The pixel size depends on what dpi you're using. Try to find out what is the greatest dpi your printer supports and post it here so we can tell you the correct dimensions in pixels. Or if you have Photoshop, for example, you can find it out as well. Actually, you don't even need to know the pixels. Just make a new image, choose inches as the measure and change the dpi value to one that your printer supports.
I think you can find out the supported dpi from Control Panel -> Printers -> Click on the printer you're using -> Properties -> General.
I think you can find out the supported dpi from Control Panel -> Printers -> Click on the printer you're using -> Properties -> General.