Cutting a Picture Mat
There are a couple options to calculate dimensions for your picture mat. Both are common, both are acceptable. Just make sure the transition from the actual art to the picture mat is smooth and the picture mat is in proportion to the frame in its entirety.
Picture Mat - Equal borders
Cut the opening so the borders are all the same size. For example, the top, bottom, and sides are all 2". For a smaller image (sized at 11"x14", for example) provide mat borders of at least 1 ½" to 2". For larger projects, consider mat borders of up to around 5".
Picture Mat - Weighted bottom
Cut the opening so the top and side borders are the same size, but the bottom is wider. For example, the top and sides are all 2" and the bottom measures 4". There are a couple theories about how and why this technique came about.
The first theory dates back to the Victorian era when pictures were hung very high up on tall walls. The angle was awkward from the ground and it gave the illusion that the bottom mat border was thinner than the others. In order to accommodate this visual deception inches were added to the bottom border making all sides seem equal to the viewer.
The second theory is that it is human tendency to feel more comfortable around substantial things. A heavier, wider bottomed mat would make the frame seem more stable thus giving the impression that it would be less likely to fall, and making the viewer more comfortable around it.