Steve Simmon's book has sizes for different camera formats but I'd wager it's only a guideline. My feeling make it big you can always make it smaller. I think I tried almost every sweat shirt I own. Not one was really light tight. My camera has clips on it's sides. Nothing on the top. Of course on the top they'd really help but four clothes pegs do the trick. Light coming up the bottom is a bit of a pain and I keep thinking I should use more pegs around the bottom. It would sort of turn the cloth into a big tube.