"The big isn't bad" is accepted as correct, but I can't think of a context where this makes sense in English. "Big isn't bad" or "The big one isn't bad" make sense, but not "The big isn't bad"... to me anyway! :)
You can make the subject of a sentence out of any adjective this way. But it stays an adjective. In Czech, you can even have sentences with "undefined" subjects (not sure what the correct term is), for example "venku prší" (it's raining outside).