I don't want to date a girl because of her size
That seems shallow. Whats wrong with her size?
She is very fat.
Although it's rude to say bad things about fat people, you have the right to date who you want to date. That is your decision.
No I want to date girls who are thin, not fat.
Thats fine if you're attracted to thin people but if you already made plans with this other girl then you better find a nice way to let her down. Just because you're not into chubby people doesn't mean you should treat them badly. If you do their self confidence may suffer.