After playing with a matrix that was very wide (126 pixels) but only 32 pixels tall, I realized that it made anything "graphical" look odd. I want to be able to display pictures of Santa and Elves and Spooks and Ghosts. It occurred to me that I really wanted a Video Screen shaped matrix. Since the two common sizes for video screens are a 4:3 ratio (standard definition) or 16:9 ratio (high definition), I played with numbers to get the right size.
My constraints were - I already bought a LOT of Smart String Flex strips, so that made my distance between pixels fixed at 33 millimeters (30 pixels per meter). And I wanted it to fit inside my Garage Door frame so that it could hang from the door header.
Since Flex Strips have to be cut at multiples of 3, that limited me also. I ended up deciding on a matrix size of 72 pixels wide by 54 pixels tall. A perfect 4:3 ratio. In xLights/Nutcracker modeling, pictures just seem to fit well. Plenty of size for text, even two rows.
Total size is about 8 feet wide and 6 feet tall. The painter's tape on my garage door was to convince my wife that it would be large enough. She was thinking "whole garage."