I like customizable sandals, because I use them as inspirations for ways to make "men's" sized sandals look totally femme.

width — No idea. I'm guessing 3/4"?
length — "ribbons average 64 inches long"
hem / finish — There are different styles. Some are clearly hemmed. Leather straps are less so. Other cotton ones are harder to see that they're hemmed, but upon close inspection, they definitely are.
material — You can look through everything that's available. I don't see any grosgrain ribbons, let alone rugged webbing. They seem to be mostly hemmed fabric.
- maybe twill ribbon would work?
dirt / moisture protection — It's possible they have hydrophobic treatments, but it's unclear.