Yes, you're right to that is awkward to say просторная щель without a proper context, still you can say like that, for example:
You want to hide somewhere, but you don't know where, and you ran across a big gap in a rock (or something else, doesn't matter), and you understand that you can fit in that gap nicely and a good amount of space still would be between your body and the surrounding edges of the gap. In other words, that won't be a tight squeeze for you, and просторный is the exact opposite to tight regarding a space.