I have a HQ16 computerized with a pcquilter. How large the quilting space is depends on: A) the size of the quilt,
B) the batting used, C) the backing used. In other words, there is no set maximum quilt space with the take up bar full or nearly full.
I don't do too many quilts that are bed size, lots of lap quilts - 60 x 70 +/- a few inches. Usually I can get around 10 inches of usable quilt space. If I use fleece as batting and backing, the sandwich is fatter, so I get less. Using cotton backing and 80/20 backing I generally get the 10 inches. Flannel backing is puffier, so less quilting space with that, too.
The maximum quilting space i can get is 12", but that is rare. and generally is a baby quilt.
Happy hunting!