Depends what you mean by one size too big. If the top tube is too long, even a shorter stem won't necessarily help. If the seat tube is too long, well, even putting the seat post all the way in won't help. Having said that, I think most people can make a bike frame that is one size too big (E.g. 56cm vs 54cm) just about work with a shorter stem and putting the seat post down. Not that I'd recommend it, but you could fit a shorter stem and twist the seatpost around and refit the saddle (so the saddle is closer to the handle bars, but it still puts your body too far forward of the pedals). I've never done that though (seen other people do it and ride a bike no problem) and don't want to do it.
Certainly not optimal, but it would be rideable. Pretty much a bike -no matter what it is -isn't a bargain if it doesn't fit you and you can't ride it comfortably (well, unless you could sell it for loads of money, read on to my next sentence). I'd consider a swap or a sell and go a frame of the right size for you.