One of the first steps in matting is to decide what size and how many layers are going to be used. Mats can be single, double, or even triple with various amounts of inner layers exposed. My typical mat is a double with 1/4" of inner mat exposed. The width of the outer (nearest the glazing) varies depending on the of the image. Here's the table I start with:
Rule of Thumb for Mat size.
United Inches Suggested Mat
8" to 11".....................1"
12" to 17"...................1 ½”
18” to 24”..................1 ¾”
25” to 36”.................2”
37” to 44”.................2 ½”
45” to 56”.................3”
56” to 60”.................3 ½”
It comes from a blog post on AmericanFrame.com's blog. Color is of course important, but I'll let that discussion alone and move forward with the steps of actually cutting a mat.