I do think it could be a sea hare. I think a major possibility is that it is Petalifera petalifera, which "...is known from records throughout the warm waters of the world."
http://www.seaslugforum.net/factsheet.cfm?base=petapeta
As for variation in size: 
(If you have a hair algae problem...)
Right down to several species that top out at about 4cm in length.
http://www.seaslugforum.net/factsheet.cfm?base=seahlist
I had one of these fellow for a while: http://www.seaslugforum.net/factsheet.cfm?base=stylstri
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-Quinn
Man, n. ...His chief occupation is extermination of other animals and his own species, which, however, multiplies with such insistent rapidity as to infest the whole habitable earth, and Canada. - A. Bierce, Devil's Dictionary, 1906
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