It's true that i'm not a expert but and intermediate skilled level rubber powered model builder/flyer. I do however bring a lot of passion into the hobby. It's just that to me the hobby is photography and model airplanes. In this video I took a 7, 6, and 5 inch propeller and compared them on the same model. I do admit that the appearance of the model could be better but I don't feel it interferes with the experiment. BTW some people say the ideal rubber motor length is between 1.5 and x2 the length of the propeller and rear peg. I think 1.25 or 1.5 the length is ideal. I since the shooting of the video now have more confidence in my rubber motor lengths and can get longer flights.
https://youtu.be/AX-E4HWCPyI. I think that the trick is to get the largest propeller and still have some control of the model.