Mat width is usually determined by a number of factors such as the size of the piece, the scale of the room where the piece will be hung, the graphic strength of the work, the mat colors, and the look desired. Standard mat width is getting wider generally. Exaggerating the mat dimensions is sometimes done for a gallery or fine art look. The mat serves as a buffer between the frame and the work of art. It is essentially a place for the work of art to be. It should relate to the work of art and enhance the work rather than distract from it. Generally, wider is better than narrower, within limits.
One of the biggest mistakes you can make in framing is to have a mat that is too small. The picture can look insignificant, crowded, or chopped off. We are glad to help you determine the width of the mat, with your input, of course.