@Dekken: "3/4 the size of the character's head or bigger" seems perfectly reasonable, so I'd say go with that. I don't think we need strict rules for the page though. If the characters you pointed out don't seem to fit the bill then feel free to remove them, the list of characters on that page is never going to be super accurate anyway so don't worry about it too much.
@takashichea: Here's some official artwork featuring Michelle and Lucy, I think it's pretty clear she just about matches Lucy's size, and Lucy definitely qualifies. Most female characters in Fairy Tail have similarly large proportions, that's why Fairy Tail qualifies as a World of Buxom.
So yeah, just use your own judgement for that page really. We don't need an absolute rule, just be sensible.