You know, a dry suit doesn't have to fit like a bespoke suit to work okay. The more extra fabric you have, the more you have to manage gas in the suit. Some things are controllable, like using gaiters on legs that are too big. Other things are more troublesome, like having boots that don't fit. The baggier the suit, the more drag it has, as well. But I have seen a lot of people buy cheap or used dry suits and dive them for a while before they decide to put out the money for something more ideal.
If the suit is too small, it can impact your mobility, and sometimes to an unsafe degree. If it's too big, you'll have more buoyancy trouble, but that can be minimized by not using the suit for buoyancy control.
Diving a dry suit is not rocket science, but you CAN get into a great deal of trouble with one. We had an open water class that started in dry suits. The students did quite well, and we had a day of fun diving off a local charter boat at the end of the class. This was the students' 9th lifetime dive. I buddied up with a girl who lost control of her suit at 50 feet and had an uncontrolled ascent, despite me hanging onto her, as negative as I could get. (You cannot dump someone else's dry suit for them!) It reminded me of the one I did from 70 feet, when I was only a little more experienced.
I guess what I'm trying to say is that you should play with the skills people have listed here in a pool, or in pretty shallow water, until you're fairly comfortable with them. And then keep your dives very conservative until you feel as though you're in control and not the suit.