Several helpful answers from The_Nephilim in the above post. I'll add a few things too.
Be mindful that projection setups still can have the side deformation issue on games that don't have special custom camera capability. Flight sims like DCS World, FSX and P3D are all ok with custom cameras, but games like Battlefield 3/4 are still going to have the barrel distortion because of how the game camera code is setup. Nthusim doesn't magically fix that sort of thing -- it's primary purpose is to calibrate the projection to the screen surface and seamlessly blend multiple projections together. It's still tied to a spanning mode such as AMD Eyefinity, Nvidia Surround or Matrox GXM. That being said I still have a lot of BF3 and ArmA II and III hours on my setup.
Half the battle with this sort of thing is having a room to build it up in. I lucked out with an unfinished basement in my case. I suppose the best place to start is the room size and work from there.
There is no exact science for the screen design. Of the 500 or so Nthusim screens out there, every single one of them is different and unique. Mostly it's dictated by the room size and the projectors used. There are however rough guidelines I go by on my personal builds. I would not go taller than 60" on the screen height. I also would not go with a radius tighter that 60". That pushes the limits of what can be done with 2 projectors.
Three projectors either forces a wider radius or a shorter screen height because of just how much a 16:9 projector is able to cover x 3. You actually get a taller projection with two projectors but three projectors are a more universally accepted spanning setup -- which makes three projectors a safer bet in design. Keep in mind that two projectors puts your edge blend right in the center of your FOV, while three projectors offsets those blends to each side. Dual projector is better suited for AMD Eyefinity builds. There are advantages and disadvantages to both configurations.
We're still working up content for the Nthusim 4.0 release. We actually have a Wordpress site that compliments the forums, but it's not public yet. Things like user made tutorials, videos and FAQs are things we'd like to add as content here but is also good to be spread around to other sites too. A lot of people learn about Nthusim from external sources like other forums or my YouTube videos.
I realize that didn't 100% cover everything but a lot about this sort of thing is more art than science. Every screen is unique and designed to the user's needs. Everyone is here to help though, so we're all available for questions anytime.
-Brad