I actually didn't know there was a dwarf species and a bigger species of this frog until after he was bought for me as a birthday present. My mom was relieved to find out he was a dwarf, because she didn't want a huge frog hopping around. I seriously wouldn't've minded, though. =D
With my pet store, they didn't say whether or not he was a dwarf. I'm not sure how you can tell the difference, aside from waiting to see how big it gets.
As for the sexing, I'm not sure about when they're young, but I read that if the throat is yellow then it's male and if it's a peachy sort of white then it's female. Mine is male, as he's spent a few nights croaking, which is very cute to hear.
They seem to be a very hardy species that doesn't require much, and mine at least, has a wonderful personality. What I find neat about him, is that he always looks like he's grinning his head off, and his eyes make him look so innocent. But, that could just be me. =D