Hello Sylvia,
I am certain the Strombus gigas you found was a true monster -
especially given that your judgement is based on direct observation
of far more specimens than I have seen.  Still . . . a specimen of 18
inches (450 mm) would exceed the known world record size by a full 4
inches!  Even Strombus goliath, the largest member of the family,
rarely exceeds 15 inches (the record is 380 mm).  In any case, it
does sound like the specimen you found would have been a true trophy,
had it been collectable!
Regards,
Paul M.