Not really sure how to explain “size” other than just giving some stats... it's been downloaded over 200,000 and has thousands of users that span over 100 countries. The Showcase is a pretty good starting place to check out some of the higher-profile companies, projects and applications that are currently using Qcodo.
There is no definitive list of core contributors posted anywhere, but the changelog is likely the best place to see where and how contributors have contributed to the core distribution in the past -- names of core contributors are listed by each entry in the changelog.
For any entry that doesn't list a name, it was submitted by myself. While it's true that most of the contributions are submitted by me, I'm actively working and hoping to change that -- and you can read the other posts on the forum where this is discussed in further detail (e.g. a recent one is Community Development).