I should have clarified....
The sizes listed are for average anemones in the wild. In public or private aquaria, things can change significantly.
Rod Buehler has a brown S.gigantea that I've seen personally, which is over 18 inches in diameter and he's had it somewhere in the vicinity of 15 plus years....
The Long Tentacled Anemone (M.doreensis) is supposed to reach a max size of approx 15-18 inches in diameter. MojoReef here from this board has told me of a friend of his who fed his Purple LTA very heavily and it completely outgrew the 55 gallon tank it was in.....it was the only occupant of the tank and was over 36 inches in diameter.
If you had an anemone in a healthy environment, and fed it heavily and regularly, it will grow well beyond what is commonly found in the wild.
The biggest anemone's I've personally seen were in home aquariums....
A local guy here in StL had an H.magnifica anemone he'd had for over 10 years and it was well over 24 inches in diameter when I saw it. He fed it a whole U12 cocktail shrimp, (raw, w/o the shell) two times a week. It was hosting 2 deleriously happy ocellaris clowns.
The others were a pair of H.crispa anemones in Indianapolis that were wild collected and were well over 36 inches in diameter.
My personal anemones are much smaller.
I've got a brown based yellow tipped H.magnifica anemone that was 12 inches in diameter until it split in Dec/Jan of 2011/2012. The clones are now 8 inches and 5 inches in diameter and I've had the original anemone since March 10th 2006.
I have a purple based purple tipped H.magnifica anemone that is about 9-10 inches in diameter that I've had since Nov of 2010 I believe.
I have a purple H.crispa anemone that is about 5 inches in diameter that I've had since 2007....it has been the slowest growing anemone I've ever kept. It was about 2.5 inches in diameter when I first got it, and it has slowly grown over the years.... I suspect that if I kept it in a tank by itself with no clowns it would grow faster.....
Nick