As others have likely noted, 42" is the most common height for layouts in nearly all scales. That being said, I feel, as many others do, that you should build it to a height you are most comfortable with. Personally, I like layouts that are pretty darn high up so I can envision myself as a trackside spectator viewing or photographing the trains. That's not always possible or practical, of course, but it is something of a goal. My current under-construction layout is about 46" off the floor, but a good part of it will be at a higher elevation, including a point-to-point On30 line that will run the 25' length of the long wall. That line will run atop a series of cliffs, etc., and should help to make the layout look a lot deeper than it actually is.