[Neveron]

You can also just make a giant-sized dungeon: rather than running around and pretending to be children in the Steading of the Hill Giant Chieftain, you're in C.S. Lewis' The Silver Chair or in Jack and the Bean Stalk and clambering about on the giant's furniture like you're the size of a cat. After all, to a Sky Giant your 6' character isn't even 10"!

Or you just put them in big rooms or, well, outside. Maybe there's an open-air area of the dungeon complex, or there's a portal to the giant's castle, or there's a Sky Giant living in the not-Underdark in a rather massive cavern. Or there's just an underground colosseum big enough for a fight with one. (As for how they got there, well, a wizard did it.)

There's plenty of ways to handle it, and as has been pointed out it's not like they're the only monster that wants big areas - Large Dragons have the same size modifier, and you probably want them to be able to fly about for maximum impact!