My team used nothing but 30x30 last year when we used the extrusion.
We relised that in many areas of the bot it was stronger than we needed.
This year, in the main frame, we are still using 30x30...But for lots of connectors, and other misc parts we are using the 20x20.
I would think that you probably make an entire bot out of 20x20, but, well, call us paranoid of it breaking
