Hi there... Sorry that I don't have the answers you want but thought to reply as these are questions I'd like to be able to answer someday soon, as well.
I'd like to learn more about the feel and the depth... Although I've seen a vid or two of someone fisting a cow quite intensely so I'd imagine it's the same for a donut (slang for equine anus), and that gaping is probably possible. Would love to feel myself slide as deep as I can into one... But not before licking it absolutely clean before I do so, though...
Maybe someone could provide details on sex toys which accurately replicate the feel to a decent standard? That way you'd get to try before buy, so to speak ..