Wacom tablets are the ones working best with GIMP since they are the ones with the best Linux support. The two things tend to be correlated.
That said, Wacom is the market leader for a reason, they make very good tablets and the lower-range (Bamboo) is really worth the tiny extra cost in the long run.
Also, wacom has patented a technology that allows them to use pens without any batteries. Most (all?) other tablet brands require a battery in the pen. Maybe not a big deal, but I do find that I have to change the battery pretty often on my Android Tablet (which uses an N-pen with battery).
Tablet-size has little to no importance unless you are a analogue painter used to large, vivid movements. Some claim that small tablets can be stressful to the wrist, I would rather say this depends a lot on how you work personally. I've had both small and bigger tablet sizes and can't say I'm prefering one over the other - the resolution of (wacom) tablets are so extreme it really doesn't matter all that much.
.
Griatch