Use controls on the Fin Adjustment Tools rollout for adjusting some aspects of bone geometry, including fins.
Choose how the numeric settings relate to one another:
Copies the selected bone's settings; these can then be pasted to another bone.
Pastes the copied bone and fin settings to the selected bone.
The bone's respective sizes.
Setting Taper to 0 produces a box-shaped bone. Higher values pinch the bone where it joins its child bone, while lower values expand that end of the bone.
Use the checkboxes to toggle fins on the respective sides of the bones, and the numeric settings to control the fins' size and taper settings.