What kind do you have right now??
How big is it when it is fully open???
You might not want to get a bigger one because they do grow and if you start off with a big one it will out grow your tank faster. They also move allot in the beginning, so if you have any coral you will have to keep an eye on it and move coral around to accommodate it.
I have a rose anemone and my clowns love it......
Hazmat