What are basic mat dimensions?
That depends on the size of the frame relative to the size of the art. There are two basic approaches to mat widths. One is to make the mat an equal width all the way around the picture – for example, 3″ of matting material on the top, bottom and sides of the image. The second approach (referred to as ‘weighting the bottom’) makes the bottom of the mat wider below the image — for example, 3″ of matting material on the top and sides, but 5″ at the bottom of the image.
In most cases the mat and the frame should not be the same width. In a finished piece this can cause a striping effect that draws the eye away from the picture. Also, it is often the case that a narrow frame will look better with a wide mat and a wide frame will look better with a narrow mat. But none of this is set in stone and each piece presents different considerations and different possible solutions. The preference is yours.