I don't think there is really a proper ratio that can be applied universally. Different people look different on horses and at the end of the day, it's all in the eye of the beholder and of course, what the rider feels comfortable with (all given that the horse is fine with it, too).
I'm 5'9" as well, my horse is 16hh and neighter skinny nor "draftlike" and I think we're a good pair. My old gelding was 18.2 hh and even I looked like a dwarf on him, but then one of my students was riding him at times, she is only 5'6" and looked a ot better on him! It's all in the proportions.