Here's a list of lizards that could live in 15 gallon long. Of course these lizards are going to each need very different set-up, here's just a species list with estimated quantities...estimated, because every keeper out there has an opinion on stocking. This just happens to be mine...
Tank laying like normal (lots of horizontal room):
Leopard Geckos (2-3), small species of Skink such as a Five Lined or Schneider (1-3), Spiny/Fence lizard (1-2), several species of Agamas (1-2), perhaps Collard lizards if you get a small species, but they usually do better in groups rather than a single individual, you'll see more natural behavior if they are together in a large tank. I've seen a group of 4 very happily living in a 50 gallon wide.
Tanks standing up on end (lots of vertical room), many branches to climb on:
Anoles (2), common House Geckos (2-3), Tokay Gecko (1, although if you can provide more room, they are happier), small Chameleons....