After doing a bit of experimentation with level files, I've found that blocks extend up right up until the start of object data; and objects until the end of the file.
However, the segment outside of the range mentioned in Pooty's post, corresponds to X = 31, which displays extremely glitchy in game (I haven't experimented to see if it functions correctly). Actually, the same is true for Z = 31 too (except where Y = 0; I suspect the glitching does still occur there, but outside the visible area), but this doesn't present as a consecutive block of bytes that appear to do weird stuff. So in practice, these shouldn't be used. Interestingly enough, aside from not being able to get the camera high enough to see the top of it, Y = 15 works no problem.
The attached screenshot shows what happens from one angle, although changing the angle of view can give different results. The blocks responsible for the glitchiness, if they were unglitched, would be a line along the very bottom of the level at the very back, similar to the line along the bottom near the front and the two midair ones.