Here is a good base to work from.
Digital camera megapixels and actual resolution (note these can differ depending on the camera):
2 megapixels: 1600 x 1200
3 megapixels: 2048 x 1536
4 megapixels: 2274 x 1704
5 megapixels: 2560 x 1920
6 megapixels: 2816 x 2112 - 3032 x 2008
7 megapixels: 3072 x 2304
8 megapixels: 3264 x 2,468
Pixels needed for a 150 DPI image (fair to good image quality)
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8x10: 1,200 X 1,500 pixels - most 2-megapixel cameras
11x14: 1,650 X 2,100 pixels - most 4-megapixel cameras
16x20: 2,400 X 3,000 pixels - most 8-megapixel cameras, maybe some 7-megapixel cameras
Pixels needed for a 200 DPI (good image quality)
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8x10: 1,600 X 2,000 pixels - most 4-megapixel cameras, maybe some 3-megapixel cameras
11x14: 2,200 X 2,800 pixels - 7-megapixel cameras, though most 6-megapixels cameras should be close enough
16x20: 3,200 X 4,000 pixels - although this is only 12.8 megapixels, most cameras won't have the same aspect ratio (width versus height), thus you may need a camera with a greater number of megapixels
Again, these numbers are just provided as the results of mathematical formulas. You can resample the image to some degree to make it larger than normal, play around with the printer dpi, etc., and get acceptable larger prints from smaller photos. Just realize that the image quality may not be optimum, though it may be sufficient for your needs.