Franco some time ago (before he became a Tarptent representative) suggested a good and inexpensive way to find out if you'll fit in a specific tent:
Go to the website and check the diagrams (excellent on Tarptent's site and several others) and get the dimensions. With masking tape and string, make a 3-D mockup of the tent, using strategically placed furniture for support where needed. Put the two of you (in my case, it's me and my dog) inside, including the sleeping bags, pads and whatever you use for a pillow (the last important so you'll know if your face ends up close to the tent ceiling, not a great idea if you want to avoid condensation).
I've tried this several times and it has worked pretty well. It's certainly cheaper than the return postage on a tent!