I just got my sebae a week ago. mine is only about 5" now, but they get BIG. I think about 20" was the largest recorded size.
I plan to keep my sebae for as long as I can, I purposly located her in a part of my reef that would not have many corals anyways (the end, towards the sandbed) so that she could get pretty large without it being a problem. I too ahve a 75g DT (the fuge/sump is a 40br, will be downsizing sump size to a 30g soon though).