Size depends on how you wanna DL it.
If you have broadband access of some sort, you might as well go for an ISO image since that will allow you to make a bootable CD.
ISO's are typically 400-650 MB.
If you're short on bandwidth, you can DL it in packages, in that case size will vary greatly depending on what you want.
As for where you can get it, there are links on most of the Linux distro's sites.
RedHat
SuSe Linux
Caldera OpenLinux
TurboLinux
Mandrake Linux
Debian
Those are the major ones, I dont think I missed any of them.
IMO RedHat is the best for your avarge workstation, Caldera's is the best for a newbie's workstation, and Slackware or Debian is the best for the hackers workstation.
For servers, either TurboLinux, or if you feel like setting up a hackers server(if such a thing exists

), Debian/Slackware.