I have a 17 inch iMac and it's great for what I need. If I had the space and cash I would get either a 24inch iMac or Mac Pro with 30 inch Display but thats only in my dreams.
It's a great sized computer, just comparatively to the 20" and 24" it looks "small". I've had one for 2 years now and it has suited me fine.... but an upgrade to 24 glorious inches is coming up soon