Love - I think how you respond depends on how much you care about your family. If it's a toxic relationship, you really do need to learn to detach, for your own mental health. If you love them and want to be around them, but just wish they would stop, there are a couple of paths you acould take, but it depends on how they are, and you know them way better than we do!
Usually when people say things mean or negative things like that it is because they are on the defensive - trying to draw attention to your 'flaws' so that the attention won't be on theirs. It is not usually done consciously, and takes some effort to change.
You could try beerab's suggestion - turn the tables on them, and comment on something negative about them. For some people this is the only way to learn that comments can hurt.
Or you could try this approach (takes a lot of practice in front of a mirror to be able to say it without bursting into tears!!!): When your dad says you look fat in 'that dress'/whatever, you could turn to him and ask him 'Why would you say such a hurtful/rude/mean thing'? He'll probably say something along the lines that he was just joking, ot that it's true, but you will need to keep on it. 'Dad, that's not funny (or Dad, I know I'm fat, and I'm working on it). It's hurtful/rude/mean, why would you say that to me'? You have to keep the focus on the fact that it is a mean thing to say and you would like an apology, not a laugh. You may or may not get the apology (depends on the person and how stubborn they are), but that person now knows you will call him on his behaviour every time he makes a comment like that.
In any event, you might want to see a therapist for a couple of sessions, to learn how to handle your family. Even if you are doing well and function normally, a good therapist can teach you how to recognize what is going on and handle those that aren't quite so 'well'.
But seriously - if you like that black dress, then wear it! Don't let a comment like that stop you from doing something *you* like - it's your dress and you can wear it. If he doesn't like it then he can't borrow it!
BP