Flat: A character with no breasts.

Small: Not flat chested, but can be from below average size to just developing. Generally in the range of A-cup sizing.

Medium: Might be considered "average-sized" breasts; B to C-cup sizing, dependent on the frame size.

Big: Pretty big, but still smaller than the bearer's head. Ranges from D-style up to the size of character's head.

Huge: Still physically plausible to somewhat unlikely. Equal or exceeds the bearer's head in size.

Hyper: From too large to physically carry (or occur naturally) and beyond.