I found that an image of 1500x2000 (Width x Height) gives a nice and sharp picture printed on A4 paper format, with narrow, even margins on either side of the page and larger margins at the top and bottom of the page with enough room for title and subscript. Divide the above figures by 2 and your printout will get less sharp, multiply by 2 and it will get sharper. Higher multiplications may result in loss of detail of color or bleeding. Depends on your printer too, i guess. Im using a HP Deskjet 930C with HP Two-sided Photo Paper, Glossy/Matte.