How big is the demo? I've just reached the Marine area, and wondering how close to the end I am.
You're close. The demo omits the last 2 zones, Space and Fortress. once you exit the Marine zone you'll get a message thanking you for playing the demo and asking you to buy the full version.
I have been told that the demo is too long, but I wanted to give people a fair chance to decide whether they really wanted to finish the game before paying for it. if a player gets to the end of the demo and says "meh, that's good enough, I'm done now", then that's OK with me.