there really are no "rules" but certain things have been referred to as rules because "as a rule....they look better (following the rule)".
If you want the look where the bottom mat is larger...generally it is half again the size of the other 3 sides. (I used to do framing aand matting) e.g. if the other 3 sides are 2 inches, the bottom is 3 inches...if the other 3 sides are 1 inch the bottom is 1 1/2 etc..