Throat size is usually expressed in height and width. So an 8 x 20 should mean 8 inches tall and 20" wide. The rollers do gobble up a lot of space, especially when toward the end of the quilting the quilt is rolled up on that roller. That's why in longarms, most people consider that the wider the throat, the better.
However, a wider throat usually means more mass, and more machine to move around. So there are some diminishing returns there, it appears...