If there was any enemies, they'd probably have the some very basic commands like walk back and forth, attack on sight, timed attacks and so on. But i'd only see enemies happening with a Mod but i'll have fun just having a really big obstacle course.
I am curious about how big we can actually make the level.
You technically can make "enemies" do things like that, but you yourself have to figure out a way of making that happen. You can give things triggers such as when something gets close to it, or something is pushed/moved and so on, but it is up to you to figure out how to make those triggers translate into an actual "attack" from an "enemy". (triggering some sort of motor to move once you get close to it, that motor might cause a monster to swing it's arms, which are hazards for example).
Basically, go and play Phun
http://www.phunland.com/wiki/HomeThat is what the level editor is like, only it has more stuff. If you cannot use Phun to make an enemy that walks back and forwards then you'd struggle just as much in LBP.
Also, I haven't tried pushing levels to their limits yet, but from what I have seen they can be absolutley massive. I doubt you will ever reach the limits on a level size (I think they are technically unlimited in size, but you can only put so much into a level but you can put in a lot).
