Well... everyone says that a SHO will "plow' with anything less than the 26. Personally... I have the 23 in the rear, all polys, ALSFB's, and intrax tokiko's with 1 coil cut up front. And my car tends to oversteer in hard cornering. I'm thinking its gotta be the cut coil making the difference.
Either way, the 19 is too small. But if you find a 23, you'll have to drill out the bushing to get it to fit. I bought a 19mm bushing, then used a 7/8" hole saw to open it up.
It would also be a good idea to see what you have up front too, just so you have something to compare it to... Front /rear ...
There was a page around somewhere and I can't seem to find it... had a lot of reviews from people who had just about every type of setup. Was thinking it was on SHOTIMES, but can't find it...
Maybe someone else here can help? If your lookin for a 26mm, look for a Gen1, most early gen 2's had 23's, but they are pretty mixed up in some years, so really you're going to need to measure them to be certian.