What is considered..."fat"?

I have often wondered, because of my overwhelming insecurities, what my "boyfriend" considers to be too fat, or if he'd think I was fat. It then occurred to me, what does society consider fat. Seriously has anyone else thought about it like this? Of course we know what we think is SKINNY and what we think is UNACCEPTABLY OVERWEIGHT (a.k.a obese) but where is simple line that divides.. a healthy weight and fat? 

Marilyn Monroe, beautiful, desired, and sexy. But, this of course was a different time a different world...a different society with different views on what women should and shouldn't look like. She my lovelies, in today's world, would be considered "fat", don't believe me? Look at some of today's actresses, today's singers, today's (role)models. A few in case none come to mind:

Now.. more worried than ever because I want to look perfect when he sees me not like the disgusting blob I see in the mirror, of course my boobs come out farther than my stomach (and that is saying something because I am an A cup) but... my ribs hardly show, my arms still jiggle when I shake them, my legs rub together when I run and I still cant wrap my fingers around them. Disgusting.