In Taanis 3:8 we learn that Choni Ha'Magel drew a circle which he stood in while praying for rain. I would like to know if there is any symbolism to his use of this shape as opposed to any other.
The Sefer Ben Yohoyada explains why Choni drew a circle based on the Gemara in Brachos 4b. The Gemara questions why there is no Pasuk that starts with Nun in Ashrei. The Gemara says that since there is a Nefila for the Jews therefore there is no Nun. The Gemara goes on to say that David HaMelech was Somech it on Ruach HaKodesh as it says Somech Hashem L'Chol Ha*N*oflim. Now since most of Adar has already passed and there was no rain yet, the Jews were like Noflim. The way to help them was by making a circle, which is round like a Samech.
The Ben Yehoyada also gives a few more reasons.
Also interesting to note that Avos D'Rav Nosson Chapter 9 says that Moshe Rabeinu when he prayed for Miriam also drew a circle. באותה שעה עג משה עוגה קטנה ועמד בתוכה, וביקש רחמים עליה לאמר: איני זז מכאן עד שתתרפא מרים אחותי
The Mabit in Beis Elokim says it was to symbolize that the Jewish people were stuck operating in a natural cycle and were thus under the influence of the mazalot (constellations). That is why Choni walks into the circle in the beginning and then when its raining he walks out of it to show that now the Jews are operating outside of the normal system.
Taken from this lecture starting at around 13:00