So far as I can see it, there are essentially two ways to choose your wardrobe. There is dressing for others and there is dressing for yourself.
Dressing for others includes such looks as the enormous flowered tent with a giant bow in the front. It means dressing to fit what people see as a fat woman image. It means conforming to what "fashion" tells us we should wear rather than what we like.
If you look at what fat women are generally being given to wear it says one thing. Please hide my body! It is designed to shame and to cloak our "dirty" and not so little, secret. Society says "Fat is Bad" therefore fat needs to disappear. It does not matter if we look like king size pillows. As long as not a single role is visible the rest of the world can pretend that we are just big skinny people. It is the beginning of a slippery slope that pushes "Fat Girls" into the margins of society.
By minimizing who and what we are it makes it that much easier to minimize what we do and what we can accomplish. It is only a small step from "Fat Girls should not wear stripes" to "Fat girls should not wear flight attendant uniforms" to "Fat girls should not be allowed to fly".
Then there is dressing for yourself. How many of us truly love mumus for anything other than, um, ok, let's face it, I can not really think of any good uses for mumus. (Yes, yes, I have heard the arguments that they are comfortable, easy, great for sleeping in, etc. but there are a number of other options out there that can fill those roles and look a lot better doing it!)
Dressing for yourself is making your wardrobe a part of who you are and giving off a first impression that tells people you want to be recognized for who you are. It tells the world "I am not hiding behind my clothes, I am using my clothes to make a statement about who I am. I am not going to let fashion dictate that I should disappear.
So wear what makes you feel good and think of it as a statement for both Fat Activism and your own psyche.