have an 03 impreza rs.gonna get 2.25 catback exhaust,anyone know what the stock size is,im guessing 1 3/4.

You are probably pretty close on the stock size. Optimal size for an N/A EJ25 is 2-1/4 like you said, and definitely not bigger than 2-1/2.

I believe that the stock RS exhaust is about 2 inches in diameter. I know for a WRX it is 2.25inches.

i believe you are right with the 1 and 3/4. you will have no problems at all getting the flanges to line up with a 2.25 inch piping. for some reason if you do a simple welding job and any local shop should be able to replace the flange
good luck, what systems are you looking at?
Pat

going to have shop cut right behind the cat and have one long pipe to muffler. 2.25.(welded)it'll cost me only 70bucks.