Exact size doesnt matter, Sly. All that is important is that they are 1:1 ratio. The higher the res you set, the crisper they will look. But as a performance rule, 512x512 look excellent, 256x256 looks perfectly acceptable too. The boards are 20m x 20m. Plus you will pass them travelling hundreds of m/s, so I doubt you will even see them really, so not worth going bigger than 512x512.