Space is really tight behind the third row. Maybe 1.5 feet on the floor behind the base of the seats and if the seat backs are reclined, it's literally just a few inches between the headrest and the rear glass. That said, we've managed to stuff quite a few duffle bags back there, a variety of strollers and even a 2-seater bike trailer for the kids.
We've had 2 4runners over the past 5 years (both with third rows) but our family of 5 has outgrown the truck. All 3 kids are still young and fit in the 3rd row fine when we have extra passengers, but we just bought a Sequoia and the increased cabin space makes for a dramatic improvement in comfort. I don't know what the max height is, but the manual does state the 3rd row in the 4runner is only for smaller riders.