I changed it from 800x600 to 1024x768
Ah, that's likely to be the problem.
Or how about an arena for 2, 4, and 8 players, another arena for 3, and 6 players, another arena for 5, and one more for 7 (total 4 arenas)?
I think it would be better to find out how many players would enter the tournament first, then design the arenas accordingly.
circular in shape
I'm not keen on this I'm afraid! Half of the screen would end up being unused, checking whether a robot has hit the boundary or not would be harder to implement, and I feel that symmetry can still be reasonably obtained with a rectangular arena.
Ryan