Using my HQ16 on a frame, I can quilt at most 11.5 inches. Reality is that the actual number of quilt-able inches with any larger harp machine is dependent on factors such as the batting, length of the quilt, as well as the size of the harp. As you roll the quilt up on the frame, the roll takes up some of the harp space. You may start out with, as in the case of my set up, 11.5 inches available to quilt with, but could wind up with less space after each roll of the frame. My frame is 10 feet long and can quilt up to 96"wide. That's more than enough for most of my quilts.
Since I don't quilt professionally and never plan to, the 16 inch throat is plenty big for me.