Yeah, it varies by person. I do hear that 5-6" of depth is typical, but even that varies a lot.
I got 5.5" and width is somewhat above average for a cis woman. (As a rule of thumb, the green Soul Source dilator is about average width for a cis woman, and the orange one is a bit bigger.)
I've never had any significant pain while dilating; not even the first time, although at that point I was on pain meds (for catheter trouble, not even the surgery itself!). The worst it's ever been has been sort of a sharp pinch that subsides in a second or two. I'd feel silly even rating that on the 1-10 pain scale.
I'm not sure what you mean by "go back." The vagina itself does tighten in on itself when not in use - which is also normal in cis women - so it's not a gaping hole or anything. In terms of dilation, depth can be lost permanently; width can be lost temporarily.
I can definitely squeeze things out or clamp down on something, and I'm fairly sure that's true across all types of GRS (unlike the other answers), because the muscles that surround the vagina are the same ones as in cis women. There's really only one good place to put the vagina, regardless of how it's created and from what, so those muscles will always be there. This is also why the vagina doesn't gape open; when left alone, those muscles tend to tighten up, even though dilation can convince them to stretch.
I do self-lubricate to some degree, including that I get more wet if I'm aroused or penetrated. I've never been with a guy, so no idea whether this would be "enough" for that.