Regarding Priscilla wearing clothes in the wrong size, I think that is mostly about her not owning or knowing how to use a tape measure.
Why would Priscilla Maass ever measure herself? She has no reason. She doesn't care that she is overweight, so she's never going to track her progress the way women might if they're wanting to get more fit or stay in shape. We know most of her clothing looks too small for her, which suggests she does not go the route of measuring herself and finding a size on a chart. We know she does not understand fractions and gets really defensive and flustered when confronted with even the most basic things having even a whiff of mathematics. (She accused Asa of talking geometry just because he was talking about the shape of the eye in terms of "top", "bottom," and "alignment.") Size charts often use at least basic fractions and a tape measure isn't a whole lot of good if you can only think in terms of whole inches.
I suspect her method of shopping is to go into stores, find things that she likes, hold them up and look at them, and say, "This looks like it would fit."
In my experience, the fatter you get, the more off you will be with that method. Being large myself, pretty much every time I receive new jeans or pants in the mail, my reaction is, "They sent me the wrong size." But then they fit. The reason us bigger ladies struggle with visually recognizing our size clothing is becasue our extra weight carried front to back, which is then spread across the front of a flat clothing item, making it look like it's for somebody twice our size as we see ourselves in the mirror or looking down.
Obviously Priscilla is pretty fat. She's as big front to back as she is side to side, possibly bigger. So an item which fits her has to be at least twice as wide (when flat) as her, which has got to be so huge that even a woman of THAT SIZE will reject it out of embarrassment.
Also the lady is small busted and in my experience a lot of these plus sized lines seem to operate under the assumption that fat = HUGE BOOBS (and sometimes huge hips). I have this issue--my waist is one size, my bust and hips are a smaller size. I always go with my waist measurement b/c I can't abide clothing which is too tight, it just makes me feel fatter.
Size C still would have been too small for Priscilla in the waist, but it would have possibly been billowingly huge in the bust area. I swear some of these places act like you've got to have MONSTER TATAS if you are plus sized, and that is just not how all of us are.