After several HO at 36" and 40", my first O-Gauge was at 44", but the next one, the one I have now, is 48" and I am so glad! The upper levels above that are at 57" and 65".
The 48" provides enough room for me to sit on a small home-made 'scooter', and travel under the layout for wiring and to reach the numerous hatches I have. For viewing, I made an observation area as well as a raised walk around a portion of the layout. I extended this walk and now is along one side and along the back of the layout. In addition, I have several small stools that kids enjoy taking around for a better view.
Good luck!
Alex