You may want to check out www.falcon-trading.com and the float planes they have. They are ABS plastic and just might be what you need. The planes are actually from Italy and it might be better getting them back home. As far as size goes, I don't have actual sizes but if I remember close most boxes will be about 5" x 24" x 4 feet. Dealing with the salt water might make the ABS the best deal.
I fly all their planes and love them all especially the Cessna. ENJOY !!! RED