Generally speaking, you can print as big as you like. I've made pretty nice A3 prints from a D70s.
There's a school of thought, particularly those who take photos for pre-press, that believes that your camera can only print at its native resolution at 300DPI (roughtly the image pixel-for-pixel, divded by three).
This isn't actually true. A 200DPI image can look good at A4. It's can usally help to 'blow up' the image in Photoshop using image size, as this provides intelligent scaling. You can get third-party scaling plug-ins that do an even better job.
A 'film buff' semi-pro Photographer I knew said that digital camera's resolutions are 'too small' and that he can scan 35mm at 'any resolution'. So for fun we both took photos of the same things on the same day - him with his pro film SLR, me with a Nikon D70s.
He was using a Nikon desktop film scanner and his 'big blow-ups' from 35mm looked over-grainy without a lot of work, and didn't show anymore actual detail than my digital photos at their 'modest' resolutions. The colour reproduction, the light and dark areas - none of these were as good as the digital photographs.
The 'film buff' argument only starts to have merit when you're talking repro-house drum scans, and how many photographers want to pay $15-30 per image just to get their photos into a computer?
If looking for big images, bget into the habit of shooting RAW, as JPEGS are more likely to reveal digital artifacts when going for large printouts.
