.65" inside diameter
&
.65" width
So I would say a 5/8" pin diameter shackle (also called a 1/2" shackle) as long as it's .65" wide at opening.

 

Awesome. Thank you so much.

 

Something like this: 1/2

 

Yes 5/8” same as the hole on a 2” receiver for your hitch ball pin.

 

As stated above. 1/2” shackle fit mine. I used these…
1/2 in. D-Ring Shackle for ATV, Black

 

The Warn shackles fit perfect

 

1/2” shackle pin is ~ 0.63”. The RMAX front opening is ~ 0.65”. Ant shackles larger than 1/2” won’t fit.

 

I personally use Columbus Mckinnon (CM) shackles for my needs at home. They are strong and cost effective. There are a lot of people who like Crosby shackles, but they are more expensive. We do a lot of lifting and rigging were I work and Crosby is always talked about - mostly because I think they are the known name. I have yet to see a recall notice on CM equipment, but I've seen a couple on Crosby the past few months.