Make the margin visible on purpose
A very narrow mat can look like an accidental sliver between the image and the frame. Try a visibly narrow version and a clearly generous version before settling on the middle. You are comparing two distinct treatments: one keeps the entire object compact, while the other lets the small picture sit in a larger field.
Use paper strips around a copy of the image to test widths. For a 5 by 7 inch image, try margins of roughly 2 inches and then 3 inches as experiments, not as rules. The larger version may suit a quiet drawing, while the smaller version may keep a brightly colored print from feeling isolated. Keep the finished frame size in view throughout the exercise.
Read the image's own border
If the print already has a wide white margin, that margin and the mat will be seen together. A tiny mat outside a broad paper border may create an awkward extra line. You might instead use a window that reveals the necessary border and a mat wide enough to feel deliberate, or discuss a whole-sheet presentation with a framer.
Check signatures and edition markings before defining the opening. Their presence may require an opening larger than the image area. Avoid assuming they can be covered simply because they fall outside the picture; decide what you want visible before the mat is cut.
Judge it at the intended distance
A small print beside a reading chair can reward close attention. The same print across a large living room may read mainly as a colored spot. A broad mat increases the outside size but does not enlarge the subject. If you need to see the image from across the room, reconsider the print size rather than relying on mat width alone.
The bottom margin can be slightly deeper if that makes the image feel more settled, but it is optional. Compare a centered opening with a modestly lower visual weight using paper templates. Leave the decision overnight and revisit it from the doorway. The successful width is the one that makes the picture easy to notice without turning the mat into the main attraction.
