The largest harp on a 'regular' sewing machine currently on the market, I believe, is the Husqvarna Viking Designer Epic. It's 12.2" from needle to arm and 5.5" high...and retails for $15K.
Personally, I'd go into counseling in order to make a better connection with a longarm if I had one just sitting around here...but that's just me.
Good luck with your dilemma.