Looks like we're in the same boat.
Im making a kart with my 14 year old. The plan is for him to be able to take his 5 year old brother on it once in a while.
After digging around the dimensions that worked for me are 30 inches wide by 60 inches long.
With these dimensions, you should be able to squeeze two medium sized kart seats into the frame, then if necessary, build up a small surround for the left and right sides. I've already built the frame from 1/2 inch tubing (using a type of box design.
Check my own build thread to get an idea of where im going with it.
http://www.diygokarts.com/vb/showthread.php?t=10798
Cheers