Yes, the "dev" packages are the development packages.
As far as hdd space, if you have a reasonably modern computer (which it looks like you do), unless you're really packed you should have enough space, you'll just have to wait while the maps compile (which can take a few hours).
Anyway, how to be tester guys?....
First, update your copy of the game often from SVN, usually with the command
svn up in the shell.
If When you find bugs, please refer to the sticky for how to file a bug report.
You can also discuss various aspects of the game here on the forum (what you do and don't like, what features you would like to see, etc.) although if you do post in this forum please use the
search first to see if it has already been discussed.
P.S. - Seeing you're new here, just FYI
Mattn is the top, lead developer here, although several of the other primary developers also post in this forum (
BTAxis, and a number of others). You would also talk to these people if you want to contribute content (artwork, making maps, adding 3D models, etc.).