You say you have a survival third-person shooter, how is it that you'll create this gameplay? Are you going to have a lot of exploration and scavenging for items (bigger environments, less linear) or just plain few resources and force the player to play smart?
I'd take a look at Flying wild Hog's Hard Reset levels, they have a nice linear environment that usually goes on spirals and revisits settings multiple times from different angles and is a nice way to have a somewhat large level that won't need to stretch over a wide area too much. Like they usually start with the player at the lower section, then he goes to the back and climbs on some walkways that spiral back over the starting area. I think it's the best way you can do this and keep the amount of actual work to a minimum.