I have a duckunder, with 48" x 48" w. clear, and it's tight when you get in there.
It's a pain. I'm 57, no back problems. But I'll admit that the low-rolling office chair is a godsend, because my main workbench is in there too. Instead of crawling in there and sitting in the workbench chair, I just roll out of the entire layout, no problem. So if that's a hard floor I think that the way to go is a liftout AND the rolling chair, you'll take it out only when you need to, and there are times when you're in and out and in and out and you're not operating, that just taking it out and leaving it out would be a nice option.