I have recently been told that they need stronger light when they get bigger. The one at my LFS is the size of a hub cap when it opens! it is also in a 500 gal reef tank. I would try it midway on the reef to start. BTW, they take a few days to open fully when first introduced. I have mine at the top of the reef about 1"-2" below the surface under 800 watts of VHO. It has grown considerably and opens up nicely every day.
Good Luck,
Frank