I got a deal I couldn't pass up on a 10" fleshing knife. But what's a 10" knife best used for? Seems that most vendors sell 8" for mink and muskrats, and 12" and up for coons, canines, etc. Would 10" be too small for stuff like foxes, coon, fisher, etc.?

No need to have anything bigger. Your only using a few inches of the knife when fleshing.

I should rephrase that. Your actually only removing a few inches of fat or gristle at a time. So a 10" knife will do the job just fine.

Thanks. That's sorta what if I figured considering that the beam is usually 6-8" wide.

Your spot on.

So why is 12" such a common fleshing knife size?

Because some while fleshing a pelt will shift from one part of the fleshing blade to another for whatever reason , mainly because it's a bit sharper or maybe a bit duller everyone has their own unique style or way of fleshing their pelts .

There isn't a wrong way as long as the majority of the fat and meat is removed as well as the grease also .
I haven't a clue how many hundreds of pelts I've fleshed over the year's and I prefer a smaller fleshing knife over one that's longer between the handles .
Then I also keep a Swedish hoof knife handy for touching up any spots I may want to touch up after the pelt is on the stretcher .