Same resolution as a normal 23/24" but bigger pixels. I never saw the point. If your iMac is mini-DisplayPort equipped, spring for a 30" panel. (Dell has refurb 3007 WFP-HC screens at http://outlet.dell.com for $749 from time to time.)
If you're limited to single-link DVI (which mini-DVI is), you might just want a 24" panel; at least then the pixels will be the same size as on your current iMac. I used to use an iMac G5 with a 19" external monitor and having different pixel pitches did look kinda janky.