Yeah, they don't sell "small" sized condoms. I mean, what guy would actually buy them?
All I've ever seen is regular size, magnum (XL) and super magnum (XXL?).
You can have a look at a specific condom website (trojan's site is cool) and see what exactly the difference is in size and shape. The magnums aren't a huge amount bigger - otherwise they might fall off - but they're much roomier in the head.
Length doesn't matter for size of condom - all condoms are very long. It's thickness (girth) that would cause you to need a larger size condom.
See the school nurse and ask for a free condom (if you're comfortable doing so). They'll be regular size. Then you could try it on at home (try it out for masturbating) and see if it feels too tight around the shaft/head. If it feels uncomfortably tight, then go out and buy a box of magnums.