It does depend on your game. Corridor shooters have the advantage that they don't need to be that big to be convincing. If you wanted an outdoor level, think of all the scenary you would have to put in. Also, it is unlikely you would be able to utilise many of FPSC's 20 layers.
In terms of length, I think approximately 10 minutes is a good minimum length. I don't think Half Life 2 levels are much longer. Also, eye-candy will slow down players while the take a look at your handy-work, that's why it takes me longer than the average player to complete a game, because I like to marvel.
Quote: "Oh, and what are those arrow things? next to the trigger zones?"
Benge's animated sprites. Judging by the placement, next to a window, they are probably light rays.
AE