My old mechanical Pfaff 262 (on right in avatar) has an 8 " harp and I have an all metal walking foot for it which works great. I also do FMQ on it. My Pfaff 2170 has a 7 " harp and has IDT built in. I also have Pfaff's Quilter's Toolbox with nice accessories as well for that machine. I like this machine for smaller things but for FMQ I like the old stocky 262 solid strength and smoothness. I also have an old black Pfaff 130 and like the 262 it has an 8" harp. For FMQ it is the best of all for smoothness for it is solid as a rock.
Just FYI, the Pfaff Quilt Expressions have a 10" harp, and the Janome Horizon has an 11" harp which supposedly is the largest made.