I thought that would post now versus a weeks ago.
Red Matter, short of being derived from decalithium(star trek:Countdown), has the property of converting a planetary object into a temporary wormhole?
Evidently it causes matter to collapse on itself, continuously feeding the collapse, until there is no more available matter to feed itself... at which point it just "winks out." Pretty much nonsense, I think... we've never seen any evidence that singularities every "go away" once created, nor is there any math supporting such a hypothesis.
How large/long is the new Enterprise?
Easily 150% the length of the "real" Enterprise... probably closer to 200%. I base this upon a number of factors, but the single most telling one is the shuttle-launch sequence... even if that bay fills the entire upper secondary hull, it would require a ship quite a bit larger than the original ship.
What speed can it attain(in TOS/TNG warp scale units)?
Totally unestablished... it went as fast as the writers wanted it to go (which, honestly, has been pretty much the case with ALL Star Trek, with the possible exception of TMP).
How could the Narada change the tech of the Constitution class ships so rapidly?
It couldn't. This was just an excuse for the folks doing the new movie to create "newer, kewler" versions of stuff because they didn't want to use what they already had to work with. (Whether they succeeded in creating something better... or failed miserably... is another topic for another conversation.)
How did the stardates before Nero's incursion become yyyy.ff?
Because that's what the guys doing this movie wanted them to be. There's no logic behind the change... it simply disregards the prior system and pretends it's explained without really justifying it. (Evidently, in Abrams' Trek universe, things are even MORE "Earth-centric," huh?)