Personally, I use THE "Q-SNAPS", not the other generic brand that I found at Michael's, was cheaper because they don't seem to hold the fabric as well and keep slipping. The "Q-Snaps" are a bit more expensive but I believe they are worth the extra cost.
As for size, a lot of it depends on what size patterns you frequently work on. I use a hoop on small projects but use Q-Snaps on medium size and large projects so I usually buy the 11 x 11, 11 x 17, and I think they have a 17 x 17. With enough different sizes, a stitcher can make a variety of different sizes to suit the patterns they want to do.