If you have the big Pokedex book for HG/SS, one of these, it will show you the differences of Magikarp sprites. The differences of spites are very small. I emphasize very. Yet the main feature to tell the size of a Magikarp is something to do with their tails, either the shape or size of the encountered Magikarp. Unfortunately, I lost my copy and can't tell what is the actual shape or to verify. This link will show you the prize of when you show the largest Magikarp. I couldn't find any codes for this Question on Bulbagarden.