At 300-400m there's no way anything we see in the movies would fit properly inside the ship. The shuttle's are large, and there are about 20 or more of them in there, with a huge hanger to move about in. That's not counting the coolant and reactant tanks, deflector assembly, etc etc Most ships in this fleet look worn and rugged, the Enterprise is the only shiny and roomy one there. Starfleet might save up resources to build fewer but much larger capital ships, leaving the rest of the fleet more function based. But that said, it's been pointed out that they'd all be 400m at least themselves. No tiddlers in this fleet apart from the poor Franklin.