Also, a XCV-330 with a 5km diameter engine would at least be the overall size of Earth Spacedock. Basically we're talking about a Babylon station sized thing.
This seems to be a bit huge if you ask me, but if it can be rationalized ...
May I recommend this BBS thread
which essentially revolves around the XCV-330 Enterprise
While I personally prefer a larger size for this ship (explaining its absence on the conference lounge sculpture wall of the E-D), the windows of the "enviropod" suggest a smaler scale.
For what Roddenberry wanted his "Starship" TV series to be, I'd dare to say that the ship would have been much bigger. Then, it ended up on the TMP Enterprise's
rec deck display telling us little or nothing about it's actual in-universe size.