Hi plantermunn,
From your description, it sounds like they are up to par for giants. Most of my zinnias range from 1-4 inches. Isolating the flower head will be the only way to keep insects from cross pollinating them. I usually let Mother Nature take care of my pollinating needs. Then I get to see what unusual combinations come up next year. I would also suggest going to the Annuals forum and read the threads posted by maineman about breeding zinnias. You will find a lot of useful information there.
BTG

BTG

Thanks. There is a lot of stuff over there.