Girls who wear the dress size rated their general happiness higher than any other with a quarter saying they were extremely happy.
More than 43 per cent of size 14 women also said they were as happy as they could be with their career, while almost a third say they couldn't be more content with their love life.
Second happiest, according to the research by Special K, are size 12 women, with almost three quarters saying they are completely satisfied with their friendships.
Forty-five per cent also enjoy a very successful career.
A spokeswoman for Special K said: "It's great to discover that being a size zero doesn't necessarily bring you happiness. Our research shows that the curvier size 14 woman is much more comfortable with their shape and has a happier outlook on life."
Size eight women came third in the happiness poll, of 3,000 women, closely followed by those who wear a size 16, and a size 10.
But those who wear a size 24 or above didn't fare quite so well, with a staggering 18 per cent admitting they are extremely unhappy with their life.
Less than four per cent are happy with their career, while more than one in ten admitted they were extremely unhappy when it comes to their love life.
Size 20 women were second in the list of unhappiest clothes sizes followed by those who wear a size 22.
But researchers discovered that it's not just plus-size women who are unhappy, as tiny size six girls came fourth, with size 18 women at number five.
The poll also revealed that 48 per cent of all women aren't happy with their weight.
More than two thirds also said they weren't their ideal clothes size, with the average woman wishing they could wear a perfect ten.
A spokeswoman for Special K added: "It's great that curvier women are happier, but we know that many women still find it hard maintaining their ideal shape.
"That's why we've launched a free online tool called Shapemate that has all the help and support women need to achieve their slimming goals."
HAPPIEST DRESS SIZES
LEAST HAPPY DRESS SIZES
Well, since my size isn't on here, and I am bigger than a size 24, I will add mine to the happiest list. Good grief, next it will be neck sizes or something. FWIW, I was way happier at size 18 than I was at size 14. I wouldn't mind being that size again at all. I could wear a size Large from all the stores and I could walk without pain. Yes, it looks pretty darn good to me.