A guy said he wouldn't go out with me because I'm big(meaning I'm fat) how can I change that?

Find someone else.

Don't change anything about yourself to satisfy someone else. If you're happy with you, that's all that matters.

I've asked out 4 people in the last 2 weeks all of them have said no.

Please don't let the fact that they said no affect how you feel. You just haven't met a decent guy yet.