You write as much as you feel you have to express about the product, at the end of the day the feedback box is there to give other potential customers an idea of what working with the vendor is like and what sort of product you can expect to receive. Sometimes I write just a small sentence to say all went ok if the vendor is reputable, but for newer vendors or those with less feedback, I usually give them a bigger paragraph where I can of their good points and message them with any tips of improvement because it helps them get started if I feel they have quality product.