Surely you can find a dive shop or 2 in the DC area so that you can try some on in the shop. That would be more practical than ordering a couple of wetsuits, trying them on, and then having to return one or both. And sizing will vary from manufacturer to manufacturer. Normally, you will need a larger size in a wetsuit than you will regular clothing. For example, I wear XL clothes, but have to buy 3XL wetsuits.
As for fit, you want a wetsuit to be a little snug but not too tight. You don't want it so tight that it cuts off circulation or causes pain. By the same token, you don't want it too loose as it won't serve its purpose, which is to restrict the flow of water between it and your skin, which heats the water inside the wetsuit somewhat, and keeps you warmer.
Good luck with your search. I agree with what others have said. A full suit offers more protection and a good versatile thickness is 3mm. I use a Henderson Thermaxx for most of my diving, which is good for both warmer ocean diving and colder quarry diving.