The largest harp on machines with feed dogs is 12 inches. The Bernina 8 series, the Janome 7700, 8200, 8900, 12000, and 15000 have 12" harps. There are many Pfaffs, Brothers, Baby Locks, Vikings with 10"+. Consider height also. Many people find the Juki 2010Q and other similar straight-stitch only machines very comfortable for sit-down quilting, even with "just" a 9" harp, because of the height.
We are all different in our abilities and preferences, best of luck finding a set-up that works for you.