Just use some pool time to learn to roll back upright, then you know you have nothing to worry about. This makes it easier to relax and concentrate on learning to use your new kit. BUT don't let anyone get you in a suit with legs clearly too long and feet clearly too big => if the drysuit feet can escape from your feet while you are upside down, you do have a problem. Drysuits need to fit though, the suit must have room for your undersuit and still good maneuverability, but it can't be unreasonably large, and unreasonably small is much worse.
You can finds lots of excellent SB threads about learning a drysuit, I won't write what I amd many other have already written a few times.
I was really scared about an uncontrolled ascent when I started. After my 1st open water drysuit dive my buddy said "Gee, you really like this quarry a lot".
Me: "Why do you say that"?
He: "Everywhere we went, every single time you saw a big rock, you swam over and started hugging it".