http://oyster.ignimgs.com/wordpress/stg.ign.com/2014/02/Guardians_of_the_Galaxy_team1-610x457.jpg
This pic was posted on IGN today of the Guardians team. (I can't get the links / attachments to work, or even copy and paste, sorry. Hopefully it works).
Anyway, Rocket's looking pretty small. I think a full sized 'mate would look really weird next to the others. It's not just about height, but also mass.
Also, Groot's not looking as big as some people are making him out to be. I know he's in the background here, but he still looks shorter than Hulk in the Avengers. I think people (myself included) thought he'd be very large because my only experience with walking trees is Treebeard.
I think a short Rocket was a good idea based on this. I wasn't sold until I saw this pic, but now I think anything else would look stupid. I'm okay with large characters being brought down to 2", but small ones brought up seems odd.