What is the shaila with the ladder with a tree shel Ashera...? The ladder must have certain Sheurim and the Ashera has NO sheur! like we say by Lulav shel Ashera is posul because Cesusai Mechtos Sheurah!
The rule that an Issur Ha'no'ah has no Shiur and is considered Kissusei Michtas Shi'urah applies only to those that must be burnt. Since burning is the only option, we consider it already to be destroyed and thus it no longer has a Shiur.
There are two types of Avodah Zorah (and Asheirah). An Avodah Zorah owned by a non-Jew can undergo Bitul by a non-Jew and become permissible. Therefore, such an Avodah Zorah is not Kissusei Michtas Shi'urah. An Avodah Zorah owned by a Jew, cannot become permissible by Bitul, thus the only option is to burn it, and therefore is considered Kissusei Michtas Shi'urah.
The Gemara in Succah (31b) explains that Mishnah's ruling that a Lulav from an Asheirah is posul, applies only to an Asheirah owned by a Jew. The terminology used there is "an Asheirah from the times of Moshe" (which the Jews were ordered to burn upon entering Eretz Yisroel), or "Asheirah D'Moshe".
In our case, the Rashba and Ritva explain that we are discussing an Asheirah which is not an Asheirah D'Moshe. Indeed, in the case of an Asheirah D'Moshe, we would apply your logic that it is Kissusei Michtas Shi'urah. The same is also evident from the Me'i'ri.
What about the Gemoro on 80b regarding Korah and Lechi?
Why does Reb Shimon Ben Lokish allow a Korah of Ashera?
Meir Eliezer Bergman, Manchester UK
Shalom and thanks for your comment.
The Gemara on 80b is clearly referring to an Asheirah D'Moshe as the Me'i'ri and Ritva (and Mishnah Berurah) point out. It is important to point out that Tosfos on 80b D"H Aval Korah does compare our Gemara to the case of a ladder, implying that they are indeed similar cases. Tosfos also explains why a Korah poses more of a problem than a Lechi, despite the fact that a Lechi also requires a minimal height.
Me'i'ri explains that according to Reish Lakish we do not apply the logic of Kissusei Michtas Shiura in the case of Korah either, since the purpose of the Korah is merely for a Heker.