Some specifics to add to JD's good advice.
Horizontal:
Fixtures are routinely spaced 18" apart. Closer is definitely possible but further apart won't
gain you much unless they are aiming far from straight out of the pocket. (If so multiply everything by 1.5 or 2) So that's 9' wide, and someone will grumble and want it to be an even 8', and then you'll lose a few inches to framing and
trim and you'll have 7'6". That might be OK but it will be tight. Tell them 10' and don't worry about the details. You will have room for a special once in a while.
Vertical:
Figure out how a tech will get their hands on the lights to work on it. Below, behind, above or whatever. That tends to take the most space and cause the most regret later. Since that will likely be you, don't put yourself in too tight a squeeze, and remember the shorter/taller person that will replace you.
Then look at the light beam. It all depends greatly on the actual angles, depth of the pocket and hanging position. Even ventilation can be a concern, as you don't' want to
trap fixture heat but also don't want to vent the room into the attic.
I always draw it out. A
line from the center of the light to as high and low as it will ever reach,
edge of beam not center. Be generous and paranoid in determining what MIGHT be needed, someday by that crazy director with a keen eye. Then widen the opening a
bit so
spill isn't too obvious. Will the the audience see the
fixture? Do you care if they do? If so move the hanging
point back and start all over, after all it's harder to reach the
fixture now.
Oh yeah, movers require a lot more space!! Not now but someday??