Remco Jan Woldhuis , Dec 09, 2007; 05:30 p.m.
You can use the Image Size function in Photoshop:
Set the width and height to the required sized (in inches) and set the DPI to 300.
E.g. if you want a 4x6 print, you'll end up with a 1200x1800 pixels image.
You can also leave the sizing to the lab, but I prefer to keep control and deliver the right size before printing.