According to the entry for this Energy Discipline Power, it is capable of having a damage ranging from 80 to 120 (depending on size). What determines the size of this weapon, and how big or small must a weapon be to correspond to a particular amount of damage? The description for this ability seems VERY vague. Thanks for any information!

For weapons, I just say that Small Arms are 80, Medium Arms are 100, and Large Arms are 120. You can find a list of Arms sizes right after the table on mundane items in the book.