The only 19x6 I know is the Bolly Carbon Fiber prop. It's very expensive, heavy and efficient. I am guessing peak 8000 rpm w/ this prop

If you go up to a 20x6, the only 20x6 that will give you more rpm is the Mejzlik hollow carbon fiber prop. It's not as wide as the rest of the 20x6 like Zinger, Pro Zinger, MA Classic, and Dynathrust[&:]

There is a negative impact when you swing too large diameter of prop on a plane too small. I tried a Zinger 20x6 on Moki 180 on a 13 lb. Cap. At idle it would rock the plane too much on the ground due to the slow rpm and large torque reaction. But we are talking about 1200 rpm idle. Not a problem in the air though.

So, like I said in your other thread, Menz 18x6 is the best for your application. For 3D, I usually go by 25% of wing span to be my prop diameter. This way the tail surfaces gets completely in the prop wash, and good portion of the wing gets in the prop blast for a good harrier.