Hey, if they can built that insanely
huge mushroom spacedock we saw in STIII, ships twice the length of the Enterprise-D shouldn't be a problem. And of course, the real-life reason is a HUGE budget increase, which led to shuttles bigger than DS9's runabouts and massive locations being used for starship engineering sections.
Technically, with the USS Kelvin being so much bigger than the TOS Enterprise (it's saucer is double the diameter and it's engineering hull longer), the question is: Why did Starfleet Prime shrink their frontline ships so much between 2233 and 2245?
We have a massive thread on the nuTrek ship sizes here: http://www.trekbbs.com/showthread.php?t=211333
, how about orthos of your finished and textured USS Vengeance?