First of all, the right person will love you for who you are no matter your size.
Second of all, if ANYONE, whether it be your dad, a friend, a stranger, etc., says that no one will want you because of ___, they are pushing their biggest insecurities onto you in order to feel better about themselves. Fact.
Maybe what your dad said is a crap, but sometimes these words encourage us to change our lives other than just get mad and cry. Trust me I’ve been called fat by many close people and that used to motivate me to lose weight and prove them wrong! And this is exactly what i did. ( I’ll be going through a 3 months journey to lose 15-20 lbs , so im here whenever you needed motivation and support)