Hey Sylvia,
 
I believe there are a few monster sized strombs out there.  I have a 12
inch Strombus gigas and it is a heavy, heavy shell.  The point however,
is that I got it from a friend (diver) who brought three back from the
Florida Keys in 1974.  The one he gave me was by far the smallest of the
three.  I don't know if his were up to 18 inches, but they were certainly
quite a few inches bigger than my 12 inch specimen.  When he picked up
his three, he said there were hundreds in the area.  Times change.
 
Tom Eichhorst in New Mexico, USA