Matt Laur , Sep 26, 2009; 01:34 a.m.
Being conservative (erring on the side of a higher quality print), assume 300 pixels per inch.
So, a camera sensor that has, say, 3000 pixels across would produce a print that is 10 inches across in that same dimension.
Rather than thinking in terms of megapixels, think in terms of pixel dimensions. The file is (natively) some number of pixels wide, and some number of pixels tall. Just divide those numbers by the pixels-per-inch of the printing device/service, and you'll knw how many inches you can go with before having to upsize the image (or have the printing process do it for you).