Plan on using fence staples to anchor snares on log crossing sets. What size would you recommend for coon, Fox, coyotes??

I personally wouldn't feel good about using a staple to hold a coyote.

Me neither.

A 2 inch staple should work well. Thorpe use that size on foot hold traps nailed to a tree root. Make sure it is good wood not rotten.

A trap or snare set in/on the ground at each end can be conventionally staked or hooked to a clog and two traps might double the take. Even set on the end of the log an extension cable or chain can be fastened to stake or clog. Those middle of the log pictures I've seen in old books are impractical, imo.
A slide wire/cable can be fastened around the log and staked away from it.
If you do use the 2" staple, chop the bark and a little soft wood off so that the staple goes into solid hard wood, then because the staple's legs twist and spread when properly driven in, you should be prepared to cut the staple arch to remove it or to retrieve the trap chain or snare extension. I have used heavy end nippers or horseshoeing nippers.
As a practical matter if using snares I think it's better to load up all the trails leading to the bridge rather than try to set on it
I don't think I ever saw a coyote walk a log or a coyote track on a log, I've always thought of that set as a fox set.

Use the ones with the barbs on them.

I would not feel confident with a staple holding a coyote, not one bit

It really depends on what you are attaching the staple to that makes the difference. I can attest to the fact that a 2" barbed fence staple has a higher holding strength than a trap stake like a wolf fang or super stake or something like that. That is however only if the staple is fastened to something it is designed to hold in and that item itself isn't going to give out or break. When I trapped in Ohio i used fence staples for dog proofs in trees and what not all of the time. The only failure i had was a tree root that was rotten and i should have known better than to ever use in the first place. It is all in how it is used and if you are comfortable with it. Take a #2 chain say two feet or three feet long and nail it to a beam in your shop or garage and try to pull it out yourself. then maybe use a hoist or come along on it. You will find it holds a lot more than you think.

I have used fence staples on coon sets, but not any K9s. They have worked well on coon.

There are several types of "fence staples" I use the ones with barbs, but have used others. IMO, I would use the barbed.

2"

Another thing to consider when using fence staples is removing them. Fencing pliers are made for this task. If you have never used them before, I suggest searching YouTube for the proper technique.

I use fencing pliers as my main trapline plier. works great for opening and closing J hooks as well as cutting chain if I have to cable as well. I use them regularly. I have not used the barbed staples but I can see why they would hold a coon just fine especially on a short chain.

Have used them to hold coyotes on a kill snare, works well. Not sure about foot holding them.

Good info. Thanks all!

If put in straight n pounded down tight , they don’t move much. I use them for coons n fishers, never a problem, I use 1 3/4”.