@Joey The Great as long as it starts the match within 18" by 18" by 18", it can expand as high as you want it too, and it can expand length-wise as long as it does not exceed 36"
basically, imagine the starting size as an 18" cube, and imagine the expansion size as a vertical cylinder that is 36" in diameter and infinitely tall
what this means is that if you had a bot that expanded to 36" by 36" square (disregarding height) then it would be illegal, because the corner-to-corner length is more than 36"
you start within 18" by 18" by 18" and after the match starts, you can expand to 36" horizontally but there's still an 18" height limit unless you are in the expansion zone
If you are not and continue to violate the expansion rule while flipping caps, it's an easy case for a dq