Hi there HorrorChix, welcome to RPF! I think most of the actresses in this are actually pretty short -- Emily Browning (Baby Doll) is 5'2" and Vanessa Hudgens (Blondie) is 5'4", for instance. But personally I think it has more to do with body type and making sure your costume fits you well.
As an example, I'm not quite 5'2" and a size 2-ish, but I've got a large bust and I'm not comfortable showing off my stomach, especially where I'm planning to wear this (at Dragon*Con, where I'll be doing a lot of partying in costume, lol). Baby Doll's outfit would look horrible on me, and accent all the parts of my body I don't like. So given what I'm comfortable wearing and what I know I feel and look best in, I'm going to do Sweet Pea instead, even though the actress for Sweet Pea is 5'8" and I'm much closer to Baby Doll's height. I know I'll look better and feel more confident in a costume that shows off my bust and my legs, but has a corset over my stomach, and I'm comfortable making corsets and working with leather/vinyl and have been dying to learn foam armor making.
Anyhow, I wouldn't let your height be a major factor in which of the costumes you make. Go with which character speaks to you, or which outfit best highlights the parts of your body you like the best, or which costume features construction you're comfortable with or have been dying to learn. No one is going to say you're too short to be Sweet Pea.
