The Gemoro (I think on 106b - sorry I've not got a Gemoro in front of me!) discusses the difference between a Bivar Godol and a Bivar Koton (re capturing an animal enclosed therein). One suggestion of the Gemoro is that in a Bivar Koton the shadows from the walls will touch.
(a) This is obviously very vague. Tosfos says that they had a Bivar of a standard wall-height, but this still doesn't explain fully what the Gemoro means. I noticed that this definition is brought in Rambam (I couldn't see it mentioned in Shulchan Oruch) - there the Lechem Mishneh explains that in the morning the shadow from one side reaches the centre of the Bivar, and in the afternoon the shadow from the other side also reaches the centre of the Bivar, i.e. the shadows from different sides would meet. (This also presumes a standard wall-height).
(b) However, I'm not sure if this totally explains the matter - surely at sunrise the shadow from one wall might go all the way across the Bivar even in a very large Bivar (similarly at sunset on the other side) - i.e. it depends on when in the day we are considering!
Do you have any explanation of this, or any other interpretation of the Gemoro?
Kol Tuv Mark Bergman
(a) The explanation that you describe is also cited by the ME'IRI in Beitzah (24a). The RITVA here in Shabbos gives two explanations. The first is the one that you wrote. The second is that the shade of the eastern wall reaches the western wall in the morning, and the shade of the western wall reaches the eastern wall in the evening.
The Ritva adds that the wall here is four Amos high, which is the height of a wall when it is otherwise unspecified (as the Gemara makes clear in the first Perek of Bava Basra).
(b) Perhaps the intention of the Gemara is that the shade of one wall reaches the other wall for a majority of the day . That is, the shade from one wall reaches the other from the beginning of the day until just after the third hour, and from the ninth hour until the end of the day (for a total of just more than 6 hours).
In fact, we may be able to understand what our Gemara is referring to from the Gemara at the beginning of Maseches Sukah. One reason a Sukah which is twenty Amos high is Pasul if the Sukah is only four Amos (long) by four Amos (wide) is because the person is sitting under the shade of the walls and not under the shade of the S'chach. That is, the shade of each wall reaches the opposite wall. Here, too, the dimensions of the size of the Bivar can be calculated according to this ratio, so that a Bivar which has walls that are four Amos high (as the abovementioned Ritva writes) will not be small enough for the animal inside to be considered trapped if the width of the walls are less than 4/5ths (0.80) of an Amah.
This seems a bit too small, though. Your question still requires further elucidation.
Thanks,
-Mordecai