There are ways to adjust headroom, but I find cabin width to be the real issue. Tall guys splay their knees out to fit in a side by side RV. The guy not flying has to work at holding one leg out of the way of the other's knee as well as clearing the pedals. Arm length is a problem too. I wear a 36 sleeve length. To fly a friend's -6 from the right seat I must grasp the stick more or less from the front, with my wrist bent at a 90 degree angle, because my right elbow is hard against the cabin wall and I can't get it back alongside by torso.
Cabin width for the -7 is listed at 40", divided by 2 is 20". You have around 30" in an RV-8 and it doesn't change with a passenger. As a bonus, your head is centered in the bubble.