Yes sheimi is the smallest, but look Manaphy is kind of the same size as these^ So that may mean that Manaphy is the lil' legendary.
BUT, now that I looked in the pokédex, I saw that all the lil' legendaries was in the end of the pokédex in their own region. Which sheimi was too but not Manaphy. This may likely mean that Sheimi is the lil' legendary...
Hey nintendo, next time, make 15 pokemon at the end of the pokedex with 100 base stats, then things will get interesting!
But really, nintendo made a fatal flaw there, i will always say Manaphy is the lil legend and its hard to argue with that if Sheimi doesnt star in a movie :/