I personally never found a good stunt prop for Cox engines aside from the 5x3. Many are too light on pitch sending the rpm's into oblivion.  I prefer the Cox props. I've used the APC 5x3 with good success on Black Widows but unlike Cox props, one minor ground strike and the prop breaks. In addition, they don't properly fit a Cox 5-40 screw. I made bushings that fit the recess of the APC prop and use the proper diameter for the 5-40 screw. Prior to that, I used fuel tubing as a spacer which is a pain in the rear. Others have mentioned using the 6x2, again, I feel it just beats air with no drive.

           It really depends on what Cox engines your using. I mentioned the 5x3 on the Black Widow which worked well. A TD doesn't like to be too loaded but likes a little pitch so the APC 4.5x3.5 works quite well I've used the 4.4x4 for speed on TD's as well. One can't just offer a props size, you need to experiment. Many are used to using 6" props on integral tanked bees which works great for the beginner in my opinion but when comes to stunt, the engine starts sagging. APC offers a 5.7x3 which again is probably the largest I would use if necessary.