Resolution for Printing

Anybody know 'rules of thumb' for the minimum size/resolution of a jpeg required to print
* A4 Poster
* A3 Poster
* A2 Poster
* A1 Poster

The reason I ask is I have a jpeg file of a poster (1653 x 2339 pixels - 200 pixels/inch) and need to know the max. size it can be printed on

The resolution you need depnds on how close your eye is to the picture. In turn that usually depends on how large the picture is: you won't normally be looking at an A1 poster with your nose right up to it.

On the Cambridge In Colour web site, there's a calculator to help. For somebody with 20/20 vision, it suggests:
350 ppi when viewing at 25cm
175 ppi when viewing at 50cm
87 ppi when viewing at 1m
18 ppi when viewing at 5m

So for a poster that you're putting on the wall, you'll probably want it to look good at 1m so 87ppi is enough - say 100 ppi to give yourself a bit of latitude and to make the maths easier. Your picture is 1653x2339 pixels so at 100ppi that would print to 16.5" x 23.4". That's almost exactly A2 size. Not bad for a 4 megapixel image, hey? But - don't look at it too closely!