Well, the Enterprise from Star Trek is fiction, so legitimate answers would include "zero actual size" and "as big as they say it is".

A true fan would probably reply: "Which one?"

An enlightened person might point out that Star Trek fans are very online, all this info and much much more than you wanted to know is just a google away.

640m / 2000 feet, or thereabouts, is also an acceptable answer.

But the unbuilt rockets in that animation were somewhat legitimate designs, a few steps from actual working prototypes; therefor those sizes had to be somewhat buildable; whereas the Starship Enterprise is FTL fiction, with warp drives powered by antimatter handwavium.