This night the 1.2 patch of the Star Citizen alpha went live to everyone.
I can't say anything about download sizes since I started the process on my machine yesterday before leaving in the late afternoon but supposedly it's somewhere around 30GB.
For that you'll get (theoretical) access to the first instance of the multiplayer area "ArcCorp Landing Zone 18".
Theoretical because right now the PTU is offline due to some bugfixing after the first stress test with some thousand players selected randomly for the different servers worldwide.
Once the PTU is back up you can load into the game as usual and should have the large hangar doors right behind you (for the R&Y hangar they're one level up). Enter these and to the right is a smaller door to a lift which will pop open an input field.
Select ArcCorp there and you should be transported to the multiplayer area.
According to folks from Neutral Security Agency there's not that much to do right now, you can basically walk around, use the AR overlay (F2) on different items in some shops and to see players names and start chatting with them (F12).