A mat does two jobs: it holds the glazing off the artwork, and it gives the eye somewhere to rest. Getting the margins right is mostly arithmetic - with one deliberate asymmetry.
Start with the art
Measure the image area, then decide how much of the paper edge you want to show. A quarter inch of paper inside the window is standard for prints; original drawings often look better with the full deckle edge floated.
Then the margins
For pieces up to 16 x 20, a 2.5 to 3 inch margin is a good default. Larger works can carry 3.5 to 4 inches. Keep the sides and top equal.
Bottom-weighting
Add 15-20% to the bottom margin. A mat with equal margins reads as bottom-heavy because our eyes perceive the vertical centre slightly above the true centre. The extra weight at the bottom corrects for it, and the piece looks anchored rather than sinking.
The frame designer applies this automatically when you choose a matted mount - enter your art size and the reveal is calculated for you.