i've never checked out etsy to see what sells- can you look at who's items have sold?
i very seldom make a quilt smaller than a queen- though i have recently made a couple twins- generally i make & sell bed quilts- i can see the (usefulness) of smaller though...on the 'Quilts for Sale' site it seems like all different sizes sell from wall hangings/table toppers right on up to kings. i think visual appeal has more to do with it than the actual size. if someone sees it & falls in love with it they will buy it- or maybe contact you to see if you could make another in a particular size they want.
i know---no help---with your situation- up to you if your current quilt needs a border- or if it seems (finished) and you can save that fabric for the next one.