To give the scale of the volume between the wings, you could handily fit three Ent-Bs in there.
Hell!! You could fit an Ambassador in there but you'd never get her out again.
But the refit Excelsiors could enter & exit easily, as long as the one amidships waited her turn.
This, by the way, is with a length of 1041 meters, not the 1200 m sometimes quoted.
Edit: Indeed, a Galaxy class could fit within the wings. It'd be a very tight fit and you'd need to cut her out, but she'd fit. Though a Galaxy-X would not...Unless the D'deridex were 1200 meters long.
Two New Orleans classes would fit, or two Springfields. (Wolf 359 vessels)
Much more interesting (to me) is that two McKinley stations would fit handily as well, if their arms were folded up. Indeed, if you were careful about how you stacked them, you might get away with four: two to starboard, two to port.
Indeed, if either top or bottom wing unlocked and hinged, you could tote some serious hardware in there, installing it ready-to-go.
With tractor beams holding your cargo from all sides, it's not going anywhere until it's time to deploy.
That would make a seriously capable and formidable beast.