if the bar is off a 03 sedan then it is a 20mm(the wagons came with a 17mm RSB). So yeah, I would highly recommend replacing yours with this one. I had a 20mm STi bar on my RS, and it made a pretty noticeable difference. Buy it, since I am assuming it's pretty cheap.
Best modification you could ever do for the price. I felt a difference immediately after I did it. You will not regret it. I got my WRX 20mm bar for $50 if I recall. It made my stock suspension come alive!
22 would probably work, maybe 24, but 26 is getting really stiff for an otherwise stock suspension. 24mm may even be a tad bit too stiff. That tail end will get extremely whip happy if you arent careful. I even had a few near ones when I had my RS with just a 20mm.