Where can I find design specs for 6mm to 22mm display signage?
There's no standard resolution for such displays. They're generally made up of a collection of individual (usually square) panels or "tiles", of say 16 x 16 LED pixels. You can construct a display having hundreds of tiles horizontally but only a few tiles vertically, like the Fremont Street experience in Las Vegas, which has an aspect ratio of about 14:1 (2596 x 184).

Plus, displays can have any shape, not just simple rectangles. Fremont Street is curved, and I've worked with diamonds, cylinders, triangles, skewed pyramids and truncated circles. 

There's no perfect way (that I've seen) of simulating the pixel grid. The best you can do is work in the pixel dimensions supplied by the display company, applying masks as needed for irregular shapes, and field test with any material that needs fine lines or small text. Assume for the most part that the intended viewing distance will allow the coarseness to 'average away'.