I have a TRX mounted with the X-Mount from the ceiling in my basement. I have 3 1/2 feet of space on either side of the anchor point for horizontal movements (suspended pendulum, oblique leg raises, side squats). This is just about right for me (I'm 5' 6", so if you're taller, maybe add a 1/2 foot).
I have a couple feet of space behind the anchor point, giving me more room to do things like low rows. I can't remember how much space I have in front of the anchor point (not home right now), but optimally you want have enough space to do a balance lunge with the TRX fully extended.
Bruce