The size on your business card will depend on your final design of the card, but should be used at 300dpi for printing. Online use (such as watermarks) will be dependent on the resolution of the photo.
If you've created it in PS, it's likely a bitmap logo, so will need to be saved at various sizes to ensure the best possible quality each time.
For a 'catch all' file, make sure it is a vector logo (you'll need to use a vector based program for this such as Adobe Illustrator), as you'll then be able to resize at will without any detrimental effect on quality.