My Green BTA, I have had for 3 years, it is approximately 8-10 inches across. He is under 2x250, about 14 inches from the light. I also have 2 small RBTA about 2 years and they also have never moved. I think it is a matter of the anemone being happy enough to stay put rather than the wattage of your lighting system. I personally would not have a anemone under anything but MH. But it is a combination of lighting, flow, heat, and other tank inhabitants. Your tank should also be set up for 12-18 months before you get one. By that time you will have also figured out all the things that you did wrong and all the money you spent doing it wrong , In that time frame you also will have most likely upgraded a lot of you equipment including your lighting.