I bought a fantail last summer and I noticed that she has a bit of a wen going. Could she have been mismarked? Do baby or young orandas look like fantails?

From my experience very young fantails and orandas look a lot alike.

wen growth occurs later in development, so yes, when young, orandas and fantails can look alike.

When should it start to get bigger if she is an oranda?

Tosakin, congrats on the baby! My sister is 4 months pregnant with her second. Are you having a boy or a girl?