Up to now the first phalanx of about any finger goes in without discomfort. The little finger goes in a further about until halfway the second phalanx, although this can be uncomfortable, but nothing unbearable.
My wife mostly uses the little finger, she is afraid to use bigger fingers since that has lead to some minor blood loss in the past. (a little red seamen) Nothing really alarming but it freaky non the less.
Probably a little toe would fit in without a problem, but because of the blood loss I think my wife would be rather reluctant to try.
As for the story how we started doing this, it all started about 2 years ago. We went to a wellness hotel for a weekend, to get some rest after the first 3 months of our youngest son. He was getting spoiled by his grandmother, and we where getting spoiled at the SPA. My wife had been joking for years about inserting something in my penis. When we were fooling around on our room she suddenly flipped me on my back and started playing with my peehole. It felt much better then I would have imagined. And in the heat of the moment I let her insert her finger a little ... and that felt SOOOOO good. The feeling didn't go away for at least 3 hours. Since then we do it rather often.
Now recording it, or getting pictures of it will be tricky, think my wife want's to keep this private. But You never know, I might get some pictures one day. And I am brave enough to post them yes.