go to google and look for a resolution chart-----inches and pixels. Now open photoshop and start by creating your file in the size you need or bigger, always easier to downsize and not loose quality, but you cannot upsize, without special software that will upsize and not destroy picture. Also if professionally printed ask them what dpi or ppi it needs to be in, again it depends on output size resolution. Same advice for Illustrator. Good luck next time.