No one size is "fat". Height, bone structure and body composition (fat versus muscle) all make a difference. A short person may be "fat" at a size 16, but a tall person in a size 16 absolutely gorgeous.
if i was a woman id be size 18 ....lol
Anything over a size 18
Look in the mirror if your not happy then change it. Some girls are happy being big some are happy being small. Some boys prefer big girls some prefer small......Simples.
Not sure about sizes. If you can run a mile, I think you are good. If you can't, then you may be fat. -_-
Unless you're emaciated and too weak to stand.
That's why I say may be fat. So many exceptions. =]