The standard newage WRX rear discs are 290mm, and they take the smaller 170mm handbrake shoes.
Its unusual to warp a rear disc though from normal driving (and to be able to feel it during braking on the road) unless its getting very hot due to a stuck caliper pistons - which is fairly common on the WRX 2 pots you have on the back. Worth checking to see if the pistons move freely back into the calipers otherwise you'll ruin any new discs too if you don't get it fixed.
In terms of recommended discs, give Alyn @ AS Performance a call or try Godspeed or Aztec Performance. Something like this will do the job fine.
http://www.godspeedbrakes.co.uk/~god...?id_product=21