IMHO, and I'm still somewhat a newb, but I did experiment quite a bit on CPs....
You can fit a full 4x2 row 2 player controls minus trackball on about a 24-26" panel.
It will still feel a *bit* cramped for two full sized Adults.
with Trackball, and giving it a bit of spacing, I'd say something like 32-34" would probably be ideal.