Quoted from SteveP3:My guess to the sizing is that if you're working with a 4x8 foot sheet of plywood, you can cut 4 playfields out of it with minimal wastage; but also avoid knots and imperfections to some degree, because of the extra space left over.
IIRC that's similar to how Jim Marshall came up with the dimensions for the classic 4x12 speaker cabinet. The cab size had nothing to do with acoustic design, it was just what best met manufacturing needs.