With Wildshape, not with Shapechange, though, right? (Away from my MotW right now.)
A cloak should be fine, though.
SIZE AND MAGIC ITEMS
When an article of magic clothing or jewelry is discovered, most of the time size shouldn’t be an issue. Many magic garments are made to be easily adjustable, or they adjust themselves magically to the wearer. Size should not keep characters of various kinds from using magic items.
and
When the change occurs, your equipment, if any, either remains worn or held by the new form (if it is capable of wearing or holding the item), or melds into the new form and becomes nonfunctional.
I can't see anything to stop a dragon wearing a dragon-sized cloak...?
-Hyp.