I agree with mystico_tala - your best bet is to use a long sleeve t-shirt pattern, and extend the bottom til it's the right length. Just make sure to overestimate the length by several inches, as the stretch tends to make the dress shorten a lot! And you'll be best off with a four-way stretch so that you can get in and out of it, and won't ride up too much.
Another option for the pattern is to get a multi-size leotard/gymwear pattern - I have one that has patterns for leotards (sleeveless, short & longsleeve, as well as two-piece) and leggings. This is really useful, because I can trace off different sizes if I'm working with a low-stretch fabric. Or, you could go cheap and easy, and trace around a long-sleeve tshirt of your own! This isn't complicated at all, as stretch fabric is very forgiving. And then all you'd have to do is decide how deep to make the front and back scoops.