2" would be fine mate. Any bigger and you will have trouble routing the pipe work. Get one of the coolers on eBay that fits the space you have. I will send you pic of the one I fitted to my rs.
The ones on eBay are what ktec sell, buy off eBay or try and find a universal forge one, I fell lucky and found a second hand golf fitment forge unit on eBay and got it for £80! just had to remove 2 brackets.