I have done aquafit. In fact I do all kinds of classes and I'm not worried about others. Only thing I worry about is my knees and I try not to hurt myself.
About aqua class - its perfect. You will see, how enjoyable it is =) Promise. Plus no one can really see you, as water hides you.
I have been in many different class in different pools and I never were only big girl in class. I have noticed one more thing. Morning classes have usually more older people and big boys and girls, when evening classes have more younger and more fit people in it.
Be brave and enjoy your workout.