Gemara

If an undeveloped area more than 5000 square Amos was not surrounded for the sake of residence, a pillar 10 (Tefachim) tall and four wide is Mema'et (diminishes the area). If it is less than three wide, it is not Mema'et.

(Rabah): A pillar between three and four wide is Mema'et.

He says so, because (three Tefachim is an important amount, so) it is not Lavud (considered connected);

(Rava): It is not Mema'et.

He holds that since it is less than four, it is unimportant.

Berachos 5b (Beraisa - Aba Binyamin): I always exerted myself that my Tefilah should be near my bed.

Suggestion: This literally means near (facing) his bed.

Rejection: Rav Yehudah taught that when one prays, nothing should separate him from the wall in front -- "va'Yasev Chizkiyahu Panav El ha'Kir va'Yispalel"!

Rishonim

Teshuvas ha'Rambam (20/215): It is good to be close to the wall, for this enables greater solitude and Kavanah. A curtain does not inhibit this. The only things that are despised are an Aron, Teivah, bags, or household Kelim and similar things that distract Kavanah. It is despised to pray facing forms, even if they do not protrude, for they distract. He looks at them and loses Kavanah. We close our eyes in prayer in such cases, whether it is a curtain or an illustrated wall.

Tosfos (5b DH She'Lo): Something fixed, such as an Aron or Teivah, it is not a Chatzitzah. However, it seems that a bed is a Chatzitzah.

Shiltei ha'Giborim (1, on Mordechai Berachos 9): We forbid only something 10 tall and four wide. Benches and similar matters are not a problem. We are concerned only for things that scatter one's field of vision, and inhibit Kavanah. Something small does not cause this, as long as he faces the wall. I saw old Rabanan say so.

Magen Avraham (OC 90:36): If so, there is no concern for Chatzitzos if one stands in the middle. (Machatzis ha'Shekel - his eyes wander in any case. If one stands near the wall, his eyes wander only if there is a Chatzitzah.) The Poskim connote unlike this. See 98:4 (brought below).

Poskim

Shulchan Aruch (OC 90:21): Nothing may interrupt between him and the wall.

Mishnah Berurah (64): Even if one stands in the middle of the Beis ha'Keneses, far from the wall, he should be careful that nothing separate. The Pri Megadim and Magen Giborim hold that if the Mechitzah is more than four Amos from him, it is a separate Reshus, and it is permitted.

Kaf ha'Chayim (128): This Halachah hints that one should not have an evil thought that separates between him and the Shechinah, which is called "Kir" (wall).

Shulchan Aruch (ibid.): Something fixed, such as an Aron or Teivah, it is not a Chatzitzah.

Beis Yosef (DH v'Tzarich): Berachos 5b connotes that a bed is a Chatzitzah. Tosfos says so. It seems that this is only for a bed in the days of the Gemara, which was for sitting. Our beds are for lying, so they are fixed. The Avudraham says that only something that separates like a bed, which is a significant place [is a Chatzitzah]. It is four wide and 10 tall. Something small is not a Hefsek. If not, how do we pray in the Beis ha'Keneses in front of benches on which people sit? Also Orchos Chayim said so in the name of the Ra'avad. The Avudraham said that the Rambam holds that Chatzitzos are things that distract. Hanging garments are not forbidden.

Rebuttal (Bach 14): Why does the Beis Yosef assume that Tosfos comes to rule about beds in the days of the Gemara? The Gemara explicitly said that a bed is a Chatzitzah! Tosfos connotes that this is his Chidush! Rather, Tosfos discusses our beds. Since sometimes we move them, they are not fixed.

Defense (of Beis Yosef - Magen Avraham 36): Tosfos discusses the Gemara's question [regarding their beds]!

Rebuttal (Pri Chodosh 21): Also in the days of the Gemara they had beds special for sleeping, like it says in Bava Metzi'a (113b). The Gemara did not answer that he prayed in front of such a bed [and it is permitted, for it is fixed]. Rather, the Beis Yosef is wrong.

Taz (1): Why does the Beis Yosef say that the Gemara discusses a bed for sitting? Aba Binyamin discussed after he rose (woke) from his bed!

Suggestion: In the days of the Gemara, the same bed was for lying and sitting. It was not fixed, for they needed to move it at the time of eating.

Rejection (Taz): When we force a borrower to sell his extra possessions to pay a debt, we leave him with two beds (Bava Metzi'a 113b). One is for eating, and one is for sleeping. Rather, this is not a distinction. Every bed is a Chatzitzah. Tosfos says that an Aron or Teivah is not a Chatzitzah, for it is needed for Tefilah in the Beis ha'Keneses.

Mishnah Berurah (65): Our beds are for sleeping. They are not normally moved. They are called fixed, so they are not a Chatzitzah. Some disagree. It is good to be stringent when possible.

Kaf ha'Chayim (129): Many hold that our beds are different, and many disagree. Ma'amar Mordechai and Eliyahu Rabah say that one should be stringent. Ideally, nothing should separate between him and the wall. One can judge for himself. If he moves the bed only occasionally, and then returns it to its place, it is fixed and it is not a Chatzitzah. If he often moves it, it is a Chatzitzah.

Rema: Something is a Mechitzah only if it is 10 (Tefachim) tall and four wide. Something small is not a Hefsek.

Gra (DH v'Lo): We learn from Eruvin 25a.

Pri Chodosh (21): The Rema erred. Just the contrary! If something is big, we can say that it is a Reshus unto itself, and one may pray in front of it, like the Teivah. Something small is a Hefsek. We pray in front of benches in the Beis ha'Keneses because they are fixed.

Mishnah Berurah (68): Also Ma'amar Mordechai holds like the Pri Chodosh, unlike the Rema. The Rambam connotes unlike them. In any case, all agree that benches in the Beis ha'Keneses are not Chatzitzos, for they are fixed.

Kaf ha'Chayim (132): Ma'amar Mordechai says that the Pri Chodosh's reasoning is why the Mechaber omitted the Avudraham's opinion in the name of the Rambam (that something big is a Chatzitzah). Lechem Yehudah asked why the Rema wrote (that something small is not a Chatzitzah) Stam, and did not say 'some say that...' Neve Shalom concluded like the Mechaber, who does not distinguish big from small. Since Poskim argue, we do not distinguish. It depends only on whether or not it is fixed.

Kaf ha'Chayim (130, citing Lechem Yehudah): The Shulchan Aruch discusses a fixed Aron of Seforim that is not moved. The Rambam discusses a movable Aron in the house for Kelim. Neve Shalom says that this is obvious.

Mishnah Berurah (67): Even if it is four wide only above but not below, it is a Chatzitzah.

Kaf ha'Chayim (131): Here we say that Chatzitzah is a problem because it inhibits Kavanah (Levush, Beis Yosef citing Avudaraham). Below (98:4) we say that it disqualifies, just like regarding Korbanos. It seems that both of these apply. If so, why may one pray facing something fixed? The comparison to a Korban is only mid'Rabanan, therefore Chachamim were able to enact like they saw fit.

Kaf ha'Chayim (133): When it is not possible (to avoid Chatzitzos), it is permitted even according to the reason that we avoid Chatzitzos because Tefilah is like a Korban (below, 98:4).

Shulchan Aruch (98:4): Nothing should separate between one who prays and the wall, just like a Chatzitzah disqualifies between one who offers a Korban and the Kli (Shares).

Beis Yosef (DH u'Mah she'Chasav veshe'Lo): I explained in Siman 90 that nothing may separate between him and the wall. The Tur says that similarly, nothing may separate him from the ground. What is his source for this? Perhaps it is due to the Isur to pray in a high place. Within three Tefachim of the ground is like the ground.

Taz (2): The Tur discusses one who stands on something at least three Tefachim tall, just like we are concerned for a Chatzitzah between him and the wall only if it is at least 10 Tefachim tall.

Mishnah Berurah (10): Some forbid even if it is less than three Tefachim. However, on Yom Kipur they spread dry grass on the ground (lest one prostrate directly on the floor, or because people are barefoot). All permit praying on this, for it is Batel to the ground.

Kaf ha'Chayim (26): The Bach and many others say that the Tur merely means that it is ideal to avoid a Chatzitzah between him and the floor. Only the Prishah forbids according to letter of the law. However, the Acharonim are stringent about praying on Kelim even less than three Tefachim, lest one fear that he fall.