You probably want at least a 10 inch diameter circle around the trackball, where no Joysticks are within it.
Buttons are much lower... so they might not to get in the way, unless they are within a few inches of it.
When you roll the ball with great force towards the screen, your hand will eventually start to arc upwards.
I believe a lot of people have made the mistake of putting the 2nd player stick, too close to a centered Trackball.
I believe they deal with it... by Boosting the sensitivity of the trackball... so that you dont have to roll the ball
as far and fast. The downside to this... is that it makes the game too easy... and isnt accurate to the way the
game is supposed to play.
IMO... it would be better to increase the control panel width, and slide the 2nd player over more... even though it
will make the Control Panel layout Non-Symmetrical.
Anyway... mount your trackball to some thick cardboard boxes... and see for yourself... how your hand & arm move
when you roll it hard. Maybe even video it from the side. It will reveal the true space requirements.
Also... you probably want almost 2ft from the ball center, to the monitor glass... least someone accidentally slam
their hands/fingers into the glass.