I think it's a pretty touchy subject. I go between a size 4 and 6, which is not plus size, but it's also not "supermodel size" or weight or whatever either. Skinny for me has connotations of people who are not only thin, but perhaps have lesser muscle definition and general body fat.
I think thin is probably the most generic term, at least for me. But I guess some people disagree.
I'd personally stay away from using terms like "normal size" in any regard, just because it begs the question of what is normal. I'm pretty sure the average American woman wears a size 12. And you don't really want to isolate anyone, you know.
I guess for you the thing to do would just try and phrase things in as positive a way as you can. I review clothing on another site a lot and there's never much controversy over saying "This dress is designed to be flattering on curvier body types and will not be as flattering on those with straighter figures." It's not a statement about what body type is better, it's a statement about how the damn clothing is built.
Conversely, if you were to write something like "This item is definitely shaped for well endowed, curvy ladies. Sorry skinny bitches, this one's not for you" THEN there might be a problem lol.
Framing the conversation in terms of body type/overall shape (hourglass, pear, apple, etc) and size (clothing sizes, not body sizes) might help keep the focus on the clothing rather than the individual.