1 - all GTI rears are 226mm as far as I'm aware. Get yourself some new pads (goes without saying) and some new wheel bearings - may as well do it all at the same time.
2 - Bleed the whole system if I were you - can you remember the last time you changed your brake fluid?
3 - Could be crap cables, readjustment, bad discs or sticking mech on the calipers. You'll know once the wheels are off