The thought has occurred to me that changing to Human size would be a good way for a dragon to find all the individual gold and silver and bejeweled objects and treasures in some community he has taken over. That could explain how Smaug was able to fit or reach into so many small spaces in Erebor to get the gold belonging to individual dwarves.
I suppose that turning into a Human or Elf or Dwarf would be even better for getting all the loot.
The bigger a great dragon like Samug is supposed to be, the more useful being able to turn Human sized or even Human would be while collecting treasure from a devastated community. And it would be handy to turn Human sized or Human sometimes, such as when eating and later digesting his meals - that would make them last longer.
But unless Tolkien deliberately wrote The Hobbit intending to suggest such ideas, I can't think of any Western culture fantasy dragons who could change size or appearance.
Fafnir, who was a man or elf or dwarf in Norse mythology changed into a dragon. Maleficent the evil fairy in Sleeping Beauty (1958) could turn into a dragon, and at last in the Teen Titans episode "Spellbound" there was a Dragon who could take on Human appearance (October 9, 2004). Could that possibly be the first shape shifting dragon in western movies or tv?