Roughly the inside of a TARDIS I would say.So what would the length be to accomodate the TAS shuttlebay, which seemed even larger?
So even thought the shuttle deck miniature was forced perspective, if you take a cross-section of it through the shuttle miniature (which was NOT forced) you should get a real-world width with no (not much) guessing.
I would expect that that would give you a shuttle deck that would still be pretty darn big.