All the content on the DLS will do for N-Scale. When you select a scale at the commencement of a project, all this does is to adjust the readings on the built-in rulers; the assests do not change size if that's what you are thinking. In any event, there are more accurate rulers to be had from the DLS.
You cannot have a 4' by 2' board. The boards must be 720m by 720m and your layout will have to sit in the middle of it. The best option is to create layouts in 1:1 scale and convert you N-Gauge plan to full size.
In the real world, I model in N-Gauge and use Trainz to prove my designs. A large N-Gauge layout from the real world will occupy about one-third of a single baseboard in Trainz. And guess what? The same layout in OO or O-Gauge takes up exactly the same amount of space.
If you are looking to do something similar to N-scale modular layouts, you could look into U-Make: http://trainz.50megs.com/
Everything is on DLS, the homepage lists the existing modules. Closest thing to modular layouts.
My Elko and Lincoln TRS is my 5.5 foot by 10 foot N scale layout expanded to fit one base board. Broader curves, longer "straightaways" and more sidings. Oh yes, and portals. It is on the DLS.
By the way, one base board in N scale would fill a two car garrage.