Why is a man criticized for being sexist when he says he doesn't like larger women, but it's acceptable for a woman to reject a man based on his height? (like saying "I don't date guys shorter than me")

Your question is actually a good illustration of why some are accused of sexism. Did you notice that you used different terms in describing men and women? "Larger women" implies fat, obese, or *ah* big boned. "Shorter" makes a statement about a particular attribute: Height.

Perhaps it's generalisations or unwitting slurs that are considered sexist.

Don't take that as criticism. I was just amused by your use of different language in describing men and women.

I have no idea, but I don't think its fair. Especially as, in your example, you cannot control your height but you can control your weight, yet those are both very real things that people say.