Does the size of your base character determine anything for Wild Shape animal size? A player I have is a pixie and was wondering if she used Wild Shape if she multiclassed as druid, would she turn into a full sized bear, lion, etc?
The size of your Wild Shape form depends on the size of the animal you transform in to, not your character size.
You [...] assume the shape of a beast [...] Your game statistics are replaced by the statistics of the beast, but you retain your alignment, personality, and Intelligence, Wisdom, and Charisma scores. You also retain all of your skill and saving throw proficiencies, in addition to gaining those of the creature.
Size is a game statistic and it doesn't say anything about retaining your original size so it is replaced.