RE: Max sail size for a board
There are a lot of variables to take into account : overall outline, overall scoop-rocker line, tail kick, max width and width OFO, rail shape & thickness (hard, boxy, soft, tucked under, etc...), fin area, fin lenght and flex, foil thickness, rider's weight/skills/energy, sail type (cambered, draft & foil shape), sail cut (roach, foot area, aspect ratio), etc...
There is already a lot of interesting posts on this topic. Please, perform a search using appropriate keywords to know more. Here is a starter as follows :