4x12 are standard size for roof mounted solar as are 4x10 and (depending on the maker) 4x16. These flow bottom to top, not U shaped. You can also often get odd sized units if the factory is in production, I was able to get 4x14's for my main roof, gave me some extra coverage at a minimum cost.

I have 2 banks, 5 4x12 panels on the garage roof and 7 2/3 4x14 panels on the main roof (the 2/3 panel is cut and plugged to go around a vent).

That gives me about 600 sq ft for a 20x40 IG. I LIKE having lots of solar - it extends my season significantly AND having more can dramatically improve the recovery time. For instance, we had rain which drove the temp in the pool from around 78 to 70. One day of moderate sun and we were back to 75 - can't do that with small spreads.

Additional panels are generally a small part of the overall budget. If you have the space and like warm water, then add more panels!