It's been my experience that the breasts are as big as the artist wants them to be.
In this case though, the only official canon I'm aware of is the bath scene in Der Film @ 26:45. [screenshot (NSFW)]
Comparing against the other girls in the same bath scene, it seems like she's small breasted: larger than Pekoe (flat), but smaller than Darjeeling (medium).
Being measured for the Ooarai Girls Academy uniform.