A lot of times when you win something, you don't get a choice in what size you get; the prize is the prize and they only have one of them to give away. I've been to rail jams where the winner got a snowboard, Salomon Salomonder I think in like a 152 well what if the kid was 6'2 and 190lbs??? Too bad. He can take it home and sell it on ebay or craigs but there is no "exchanging it for a different size". I've won hoodies and stuff and received size Mediums when I'm pretty obviously a L or XL guy. Too bad. I sell it or give it to a friend.
So that's to play Devil's Advocate.
If you *were* given a choice, and they aren't honoring your choice, please post the name & details of the shop or website & contest so that others can be informed of their shitty customer service.