Well, its size definitely seemed to change (at least in relation to buildings, people, etc.) But hey, when did believability and monster movies ever coincide. That's the beauty of fantasy.
The initial news coverage showed only one monster, but that doesn't mean that a second (or third or fourth) one didn't arrive in the hours afterwards. It's possible. If the thing seemed to change shape during the course of the movie, how about a Marvel No-Prize answer for that: perhaps as the military kept scoring hits on it and blowing pieces off, the parts grew back differently, with modifications that were more suited to the environment and conflict. Like some sort of super-adaptoid. ;p
I would be extremely surprised if they used a different CG model through different parts of the movie. I always got the sense it was the same, it just moved so strangely, in a very contorted way, and had so many weird limbs and joints and tail(s) and stuff... Hard to keep up with it. Yes, very Lovecraftian in that it was kind of beyond explanation. I need to see this again this weekend or something, I can't stop thinking about it, so damn good.
to me it looked like a wingless bat, it have the long front limbs just lose the membrain and earysa and thats what a bat would look likeafter you add a tail and 2PREHENSILE DICKS, if you didnt notice watch it agian the monster hand 2 dicks and there all squirmy. i thought they were tentacles but it had balls. AND theyre making cloverfield 2 mes think, maybe just the same attack froma different new yorkers point of view but the def franchising this baby...theres already a manga coming out that takes place during an attack in japan thats happening at the same time as the newyork one