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.