trial limitations are most likely the usual
-max 10hours or 10days (whichever expires first)
-no trading or dropping stuff on the ground (prevention against goldsellers)
The client itself is pretty small (<1mb), but once you log in it'll start downloading the basic stuff (few hundred mb). After that the client downloads as needed. If you want everything installed and uncompressed, change the shortcut of the game to "gw.exe -image" and run it once (>3.5gb atm, this downloads stuff you'll never see). To play remove the "-image".