In framing design, there are very few "rules", although there are some guidelines. In general, the best mat border width is determined by a number of factors, including the overall size of the piece, the width of the moulding, and the particular hue and value of the mats and the art. In general, the goal is to attempt to give the art enough "space" inside the frame to allow the eye to rest upon the art without being distracted or crowded in by the framing.
That being said, mat borders nowadays tend to fall in the range of three to four inches. Although mat borders of two inches were common some years ago, the accepted width has been growing in recent years, in part due to the increasing width of mouldings. In most cases, you will want your mat to be appreciably wider than the moulding: A mat that is much narrower than the moulding ends up looking like an afterthought, or a way to force a picture into a frame of the incorrect size, while a mat that is the same width as the moulding can be visually unsettling; it is more attractive to have some variety in the design, and avoid the visual monotony of repetitive widths. As a rule of thumb, I like to shoot for a mat that is approximately 2/3 of the total width of the combination of mat and frame, (or about twice the width of the frame). For a two-inch frame, this would give you four inches of mat border as a basic starting point.
