We just compress our bodies like cats.
Any hole we can get our big heads through is big enough. :)
I'm 6' 2" tall, with 2' wide shoulders and average 200 lbs.
Thank the good Lord I don't have to do it any more, but I've crawled through a lot of manholes, snaking my way through double bottoms of ships, and between tank baffles, all while carrying welding, cutting, gouging and grinding equipment.
It's all OK, except for the gouging. When your stuck inside a small space, gouging is no fun at all.
What really stinks is when you get all the way in there, and get set up, and after you realize you forgot something crucial, and have made a second trip to get it, and then as you're finally about to get started, your light bulb breaks.
If anybody gets stuck in a man hole, They'll most likely never do it again because their "Buddies" will generally take unfair advantage...