The best choice is to exactly match (or exceed) the resolution of your final movie. For example, if you were looking to make a 1920 x 1080 HD video (16:9), then you would want your pictures cropped to those exact dimensions too (1920 pixels wide, 1080 pixels tall).
The most important thing is to match the aspect ratio (16 units wide and 9 units high) if you want to avoid black bars.
Width = 1920 = 16 units x 120 pixels/unit
Height = 1080 = 9 units x 120 pixels/unit
So in the example above, you could crop to 16 inches by 9 inches where your pixels/inch was 120 ppi.