Lycanthropes in general can shift between their humanoid, animal, and hybrid forms. In humanoid form they are Medium size and in animal form they are whatever size the animal normally is, but there is no clear indication on what size the hybrid form is.
A wereraven would be Tiny in animal form, since ravens are Tiny. Would the hybrid form be Medium (like the humanoid form), Small (halfway between the two), or Tiny (like the raven form)?
The wereraven's statblock is in
Curse of Strahd, p. 242
and it lists the Shapechanger trait:
Shapechanger. The wereraven can use its action to polymorph into a raven-humanoid hybrid or into a raven, or back into its human form. Its statistics, other than its size, are the same in each form.