Fat is a bad word, unless you want to make a joke of your very good friend, or somebody completely unrelated to you in your everyday life.
To be courteous, you can say chubby if you refer to his/her face. If you talk about the body, big may be an OK word. But big may also imply tall. If you have to say fat but short, how about stout? Plump may be OK too. Another way to circumvent this embarrassing comment is to use a negative sentence, like "She's not skinny". People will understand you.