If the big is too big, it's not going to sit properly in the horse's mouth. It's going to slide around and be generally obnoxious for the horse. There's no shortage of used bits out there - do your horse a favor and stick to the right size. The only kind of bit where it might not really matter would be a solid bar bit like a mullen mouth, because it doesn't have joints or a port that will be annoying to have out of place.
* I'm often reading and posting from mobile and Siri loves to make a mockery of the English language.