My two DCs have a bedroom each - DS, 10yo, has a room almost as big as mine and it's a tip. DD, 7yo, has a room half the size and she keeps it reasonably tidy with a bit of reminding.
Part of the problem is, I think, that there's simply Too Much Crap in their rooms, DS's in particular (loads of old toys he doesn't play with plus mountains of books and comics), but he refuses to part with any of it. We've got toyboxes and so on but it doesn't really seem to help - if anything, all it does is encourage him to hang on to even more old beloved playthings junk.
I'm seriously tempted to say to him that if he doesn't keep his bedroom reasonably tidy between now and the summer holidays, he's swapping bedrooms with his sister so she gets the big room and he gets the smaller one. It would likely cause him to sulk for weeks, though.
Or does anyone have any ideas how I can incentivise, cajole, threaten or otherwise brow-beat the little sod darling into keeping things tidy enough that you can at least see most of the carpet? They're both fantastic kids most of the time but the state of bedrooms has become An Issue.