I'm not too sure about file size for 4K, but for 2K, it's generally about 1.5gb per minute.
The thing about the 4K projectors is that movies that are mastered in 4K are few and far between at this point. Of course, as they become more popular, so will 4K masters. But at this point, the majority of the movies that are being shown on these 4K projectors are only 2K.
The Wizard of Oz was a little shady, because a lot of the theaters were using the video projectors that are used for the ads before movies. Most of them are probably not much better than yours.
But in general, a BD on a 4K projector should look pretty damn good. I've seen a number of BD's on 2K digital projectors, and was highly impressed. I would say it's comparable to what BD looks like at home, only way bigger.