I was tentative about it as well. I read up on it, and watched a couple YouTube videos. The first roll was outside the bag, the second inside, the third for real. You'll be fine after a couple practice runs.
I don't know the minimum size for 120 film. My philosophy was to buy large enough to work with 8x10 sheets if I needed to. You never know how large you'll need to go. . Mine is 36"x36".