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Just make it the right size for the image!
That’s sort of good advice, but it’s like saying that the way to do everything right is to not do anything wrong. It doesn’t help much.
About 25 years ago I spent considerable time working out a series of print/mat sizes that has worked for me ever since. I started with a 16x20 fiber print and decided that it needed about 4” of mat around it. I have never liked the look of having unequal borders (except at the bottom) and I wanted a modular approach that made a vertical image the same frame size as a horizontal. I sell my work at art fairs and I carry 30 to 40 framed and 150 to 200 matted prints to shows and I needed consistency.
Sparing you the boring details of how I arrived at the size, I make the horizontal prints 19.5”x15” and the vertical prints 15.5”x19”. The dry mounted print has 3/8” of mount and 3 7/8” of mat showing on the top and sides and 5/8” of mount and 4 1/8” on the bottom. This allows me to sign the mount in the 5/8” area and the wider bottom mat “visually” matches the top and sides. Both horizontal and vertical mats are 24”x28”.
When I jump to 20x24 or bigger I add an inch to the mat width all around. I ignore the minor issue of a 20x24 being a slightly different aspect ratio than 8x10, 16x20 or 24x30.
Below is a digital version of my method. The blue is the wall.
Jerome