Dependant on height, as tomcruise pointed out. Size 7/8, for example, is a definitite 'medium' as far as clothing sizes go.
A girl who is a size 6, at a height of about 5'6", probably weighs in the neighbourhood of 120 lbs - but keep in mind however that it does depend how that weight is distributed on a girl's body, and how well-proportioned she is also.
Perhaps the ladies can better advise here.