I''m not sure on the legal sizes but I asked a similar question when I 1st got my x11, at the time the trakpowers were the only cells available and they are slightly larger than the legal requirement, you may need to change to a low profile servo to give a bit more space.
I now run with stick packs as does almost everybody now, If you've got stick packs available to use I'd consider changing the car to fit these as the car handles better with them but saddles are fine.
There are a few photos in this thread hopefully if will clear up a few things
http://www.oople.com/forums/showthread.php?t=15394