Digital pictures come in a variety of sizes. That is, in electronic sizes, which are measured in bytes or millions of bytes. Picture sizes are also referred to in pixels or mega pixels. The camera often has the ability to format the pictures before putting it on the camera memory chip, producing JPEG files of differing size and quality. And then there is the printing of pictures where Dots Per Inch or DPI becomes an important consideration. This makes it all somewhat confusing.
GIMP has made it easy for the average user. There are two distinct options when you go to the top menu under “Image”. For sizing your prints you select the obvious “Print Size”, and for scaling your electronic pictures you select “Scale Image”. The Print Size selection comes by default in Inches. Images don’t always come in the exact proportions that will fit the printer paper you have chosen. So select the most important size. The excess will be trimmed off by the printer automatically, or you can cut the paper afterwards to match your printed image before mounting it in an album or frame.