What length skirt should I get to go with a garter belt and stockings? Perferably tight fitting.?

Since you're a dude- most guys won't care about the skirt- only the fact that you're cross dressing. If you are going to be around people who expect that or are interested in that, then I would say just long enough to cover the top of the stockings when you are sitting. That way there's an opportunity to tease if you want.