It's unclear whether Sarah's current form indicates how she transformed someone else without warning, or if she's just that size to fit in the bird cage, but it's safe to say she broke party rules!
That's supposed to be Nanase's foot in the back, but hey, I'm not the boss of you. Imagine that's whoever's foot you want.
I actually made a 3D model of that gosh darn bird cage to help me with this. It used to have more bars, but after accepting the horrible, horrible truth that seeing Sarah was more important than the bar spacing making sense, I removed half of them.
For once, I'm not completely sure how big Sarah is, but the bars of the cage were only 15 inches (38 cm) tall in the model, so... Pretty small.