It depends on your screen, most likely 72 dpi, 96 dpi for expensive LCD ones, 120 dpi for extremely expensive ones. (dpi = dots/pixels per 'simulated' inch)
For screen resolution output, use this.
For printing, 150 dpi minimum, 300 dpi for standard 'photo/magazine cover' resolution.
In photoshop, you can go under Image > Image Size, and use the drop boxes and blanks to give the size (in inches you want,) and the desired dpi.
if it looks crappy, and at 100% size, you have to drop down size or dpi, because the original isn't fit for better; if it looks crappy and LESS than 100% size, zoom in to make sure it's clear... if the file actually shrinks to fit your new ratio, you started with an excellent image, and will now have an equally excellent output/print.