But, don't make them the same size.

A framed image is a small world of its own. Multiple elements-frame, glass, image, mats, backing-with different reactions to temperature and humidity must fit together.

For convenience, the framing industry uses the glass size as the reference point. The glass, mat outside dimensions and backing are all the same size. The print must be the same size or smaller. The frame must be approximately 1/8" larger!

Why? Except for the glass, all the other elements are usually made up of some sort of paper. They expand and contract with changes of temperature and humidity. If the frame is too small, the sides of the other materials can expand to the frame edge. If it continues to expand it has to go somewhere-it buckles. Ever see a frame with concave mats? The frame was too small or the materials inside too large.

Don't go overboard though-you don't want the glass to fall out!