Say you have 8 megapixel photo. This is 3504 x 2336 pixels. You wish to print A4 11,68 inch wide. Then 3505/11,68 = 300 dpi.
Conclusion: print of 8megapixel photo printed at 300 dpi is max. 11,68 inch wide.(photoshop: select image pulldown menu, select image Size.