A 'young adult' male--up to maybe 12-13" or so--would be about the largest I'd maintain in a cage of this size, but for an older and larger adult male (15"+) something larger is definitely preferable IMHO. Adult male Veiled are large chameleons, and definitely appreaciate space to roam, which would be infeasible in cages of this size. If a 24x24x48" cage, or something along those lines, is not feasible for some reason, I'd suggest that a smaller species (or female Veiled) would be a better choice. There are lots of smaller chameleon species that would do well in an 18x18x36" cage though.
They're kind of overlooking the second part to your question. The 100 gallon is 48" high. And it's more than the correct width, just slightly smaller in depth. It is a little smaller than the standard in overall square inches, but I dont think ts by so much that it wouldn't be acceptable for an adult.