What was 60 Amos long?

Rashi: The Heichal and Devir (Kodesh ha'Kodoshim) together. Radak - the Heichal was 40 Amos long, and the Devir, 20 Amos.

What was 20 Amos wide?

Radak, Malbim: Both the Heichal and the Devir were. The Devir was square, 20 by 20.

How did the Bayis' dimensions compare to those of the Mishkan?

Gra, Malbim: The Bayis was twice the length and width of the Mishkan (which was 30 long and 10 wide), and three times its height. The Mishkan was 10 tall, to hint to the 10 levels of Kedushah
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; the Shechinah never descended below 10 [Tefachim above the Reshus of the land]; the Aron was nine Tefachim tall, and the Kapores (cover) was one Tefach thick. The Bayis was three times as tall, to hint to three worlds above in Kodesh.

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Kelim (1:6-9) lists 10 levels of Kedushah of place - Chutz la'Aretz, Eretz Yisrael

What is the significance of the dimensions of the Bayis?

Malbim: Its volume was 36,000 [cubic] Amos, which is 18 Mil
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. A Midrash says that the Beis Mikdash above is 18 Mil above the lower Mikdash, like the Gematriya of "v'Zeh Sha'ar ha'Shamayim" (Bereishis 28:17).

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I do not understand why linear distance is connected to volume. (PF)

Here it says that it was 30 Amos tall. In Divrei ha'Yamim II, 3:4 it says that [the Ulam] was 120 Amos tall!

Radak, Malbim: It was 30 Amos until the roof, and 90 Amos of Aliyos on top of it. 'Rashi' (Divrei ha'Yamim II, 3:4) - "Tavnis ha'Ulam v'Es Batav v'Ganzakav va'Aliyosav" (Divrei ha'Yamim I, 28:11) implies that there were Aliyos on top of it. Radak - the verse implies that only the Ulam was 120 tall, unlike Chazal (Midos 4:5-6), who imply that there were Aliyos over the entire Bayis
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. Malbim citing Mahari - the entire Bayis was 120 tall. Malbim (Divrei ha'Yamim II, 3:4) - the verse taught so only about the Ulam, for only one who stood in it saw this; there was no lower ceiling above him. One who stood in the Heichal saw only until the lowest ceiling, i.e. 30 Amos.

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Radak (Divrei ha'Yamim II, 3:4): 'Ulam' is not included in 'ha'Bayis', and not vice-versa.