There is a lot of confusion concerning what the best method of preparing a digital image for printing. This should give you an idea of our approach to making large prints from digital photographs. I've written an article on Have Camera Will Travel which explains printing resolutions in a bit of detail. You can read that article here.
As a rule of thumb, I calculate a minimum resolution for any print size at 180 dpi. 300 dpi isn't necessary to obtain a great print.
I'll accept images in a number of file formats. JPG, TIFF, DNG, PSD, BMP, PDF.
Best results for high resolution prints will come from TIFF, but JPG files will suffice for just about any application.
Based on the megapixels of your image, a rough approximation can be made as to how large a print you can rationally expect your image file to reproduce.
A general guideline would be as follows.
6 megapixel image files will reproduce a high quality print up to 11" x 14". You can print larger, but image quality will begin to suffer as size increases.
8 megapixel image files will reproduce a high quality print up to about 16" x 20".
10 megapixel image files will reproduce a high quality print up to about 18" x 24".
12 megapixel image files will reproduce a high quality print up to about 20" x 30".
15 megapixel image files will reproduce a high quality print up to about 24" x 36".
Anything print larger than 24" x 36" will normally require a very high resolution camera, between 16-24 megapixels.
Of course, I'll make any print at any size from your file, no matter how many megapixels your camera has, just keep in mind, you'll be losing resolution on very large prints and it is a trade-off you'll have to make.