The gullet is measured from the conchos in front on the inside - not the outside. The seat is measured from the back of the horn to the stitching on the cantle (in a straight line).
As far as the bar size, it is really relative and by rule of thumb. A 7" gullet is typically FQH bars, 6 3/4" is Regular QH bars, 6 1/2" is Semi QH bars. Those are relative sizes since there is no set standard and every maker has their own ideas on the angle and gullet.