Well, first, printing *how*? Are we talking an inkjet hooked up to your printer or a service bureau? If an SB, contact the SB and find out what PPI resolution the images need to be in. Always consult with your printer to make sure you're submitting files in the correct format, etc, to them.
Physical number of bytes are irrelevant. Physical number of pixels is relevant.
Let's say they need 300ppi images to work with. A 4" x 6" image would need to be (at least) 1200x1800px [1200/300=4, 1800/300=6]