I am in Central FL....it's starting to get warm. I have my hive in a spot that is shaded late afternoon, which I hope will help. My entrance is 3/8" and I was thinking of making it larger until I found this post. This is a package that was installed about 49 days ago, 3.13.13. There are a lot of bees at the entrance...cleaning and fanning. I don't see any dropped pollen but there is definately a traffic jam. Do you leave yours 3/8" all year round?
FYI we are talking height and not length. I suspect you would need to open your entrance reducer a bit, not increase the height of your entrance.
My entrances are 3/8" tall.