If this company wants to lose out on sales to company which accommodates a size 6+ company let them do it. If this company finds they make more money, somehow, by excluding size 6+ sizes the will of the people has been expressed by the vote of their dollars.
The ads which this company generates are shockingly disgusting, and some that I have seen appear to be criminal and appear to use minors in a way which should be criminalized. It is bad for society to be exposed to ads like these, and that goes doubly for children in our society. -- This is a much more pertinent reason to boycott this company. If the owner is guilty of sexually harassing women that is also a good reason to boycott.
There is nothing this company could do to persuade me to purchase any of their clothing [A: I don't like it, B: their ads].