And here is a quick and dirty approximate size comparison to ponder.
In terms of dimensions then try these.
Hercules-class star clipper
L.O.A. = 750.560 ft. (228.770m)
W.O.A. = 320.564 ft (97.707m).
H.O.A. = 189.892 ft. (57.879m)
Vanguard-class star cruiser
L.O.A. = 707.906 ft. (215.769m)
W.O.A. = 394.945 ft. (120.379m)
H.O.A. = 215.439 ft. (65.665m)
Size can be misleading and most particularly length. The clipper is the second longest of the ships in comparison yet it has the least amount of interior space due to it's configuration (and it's the narrowest and shortest in height) and the bulkier machinery and systems of its era. Nacelle size is also deceptive. The nacelles of the ships in comparison aren't that different in overall size and yet the clipper is definitely the slowest (though not really that much slower than the star cruiser) and yet both are significantly slower than the TOS era ships.
If I had to guess in terms of deadweight with the Constitution
-class being 190,000 then I'd peg the Vanguard
-class at about 120,000-140,000 tons and the Hercules
-class at about 110,000 tons.
There is a conceptual evolution at work here, but there's a missing link (and maybe two) between the Vanguard
and the Hercules
I haven't worked up yet. The class preceding the Vanguard
could be very much like my original drawings for this design.