I've stayed at a capsule hotel once, and I found it to be about 3 times as spacious as I was expecting. They're really quite roomy. I think you'd have to be more than a little claustrophobic to have trouble there.
Have stayed in one, near Shinjuku station on a Friday or Saturday night. Ceiling was higher than I'd expected, and there was room to stuff a medium-sized suitcase at my feet. I was pleasantly surprised that despite location and day, it was not noisy. I got a good night's sleep.
I've stayed in several. They're actually pretty nice, but if you go, DO NOT bring luggage! Generally speaking, you get use of a locker for your personal belongings. That's it.
Although it looks like a behive, it's amazingly comfortable ! and japanese people don't usually speak so it is possible to sleep well. Just bring earplugs because of coming and going. The shower beside was clean. So it's something to do in Tokyo...