These are all relative, idiosyncratic, aesthetic judgments. What are the characteristics of beauty? You can say that beauty's organization seems unified and that beauty imparts a feeling of transcendence. How to achieve that?
Have you looked after books on design, say, home design, books about how to dress or what to wear? There, you'll find examples of what shapes, patterns, textures, styles and colors go together. Though you aren't likely to find hard and fast rules, you'll find what the authors learned in art school and in making a living. You could go to a museum or a gallery to see how they present their photos. You could seek out someone who mats images for a living. They could tell you how they make a living at it, what they like and dislike.
When I choose a mat, I either want to make the image stand out or make the whole framed construction stand out. Sometimes I like it matching, sometimes contrasting, sometimes bold, sometimes understated. You have to learn when one or another is called for and to what extent, same for frames, same for fitting it to a decor.
You wouldn't want the photo to become lost in a sea of surrounding paper. A suitable border sets the photo off, separates it from its surroundings. For a B&W 14x17" print, a 3.5" border sounds OK. Depends on the image, depends on the message.
