I'd come down hard on this @iCantStopppEatinggg it's not ok for her to speak about anyone that way let alone her mother and it's certainly not ok after both parents have told her it's hurtful and mean. This is not what roasting is and you need to make that very very clear especially before she hits the bitchy teenage years and your younger DC thinks it's ok to copy her. Don't be afraid to let her see you cry and how upset you are at her words.
Nip this in the bud now and teach her that if she wants to say something about someone's appearance she first thinks 'is this something they can change in 5 seconds?' If you have food on your face then yes she can say it, if its something about a person's appearance they can't change in 5 seconds like weight then no she keeps her mouth shut and says nothing.
I'd ground her and arrange activities in the house for the younger DC that the ten year old isn't allowed to join in and make it clear to both DC the reason the outing is cancelled is because of older DC not being kind with her words. Alternatively can either you or DH stay home with eldest and the other take the youngest out?
As for the new clothes, I assume you have a couple of weeks/28 days to return? Keep them and in a couple of days when you're hopefully not feeling so raw try them on again and see if you still feel good in them like when you initially tried them on. Maybe when older DC has learnt a lesson she can help you pick out some new clothes that flatter your shape and style and see the power that positive words can have as well
Edited to add, also a good opportunity to have a wider conversation about women building women up, not tearing them down especially with the amount of misogynistic crap she will face as she becomes an adult and navigates relationships jobs etc.
Re Edited to add, have a VERY strong conversation with your D(ickhead)H and tell him he needs to step up here as well, laughing along with her and dismissing how hurtful her words actually were? No that's NOT ok and he needs to have your back on this all the way, see my previous edit about misogynistic crap, this shouldn't start at home from her dad ffs!