I’m also looking for a solution to this. The only work-around I can think of at the moment is creating a separate gallery for photos that have “large and medium” size prints available, another gallery for photos that have “small” prints available, another gallery for photos that have “small and medium” prints available… it’s a bit clunky…

It would be great if each photo was added as a ‘custom post’ rather than each gallery being the ‘custom post’.