Customer complained about size and wrote bad review

Hey friends! I wanted to ask for some advice. I was corresponding with a customer last week and they were asking about sizing etc after they had ordered an item from my shop so I was sure to not accept it until they were extra sure of the size. The listing had exact measurements and i confirmed the size and country of size.

Because of the back and forth i also bumped them up to extra fast shipping. This morning they messaged me saying they want a refund and the item is too small and a few hours after they left a 1 star review. This shop was my dream and i have had flawless 5 star reviews i've worked so hard for. What should i do?