Added to that is the matter of Torque.

The taller the prop (diameter) the more torque you will get. This gives you Climbing power. An 11 x 6 prop will give less speed than a 10 x 7, but it will give the model a better climb rate.

It's all matter of what you want the model to do.

I fly electrics as well as glow and on my favourite electric I chose between a 6 x 4 or a 5.25 x 4.75. dependig upon what I want the model to do and where I am flying it.

The latter prop gives more speed but less overall flying time on the battery.

When flown with the former prop, the model is slower in a straight line, but gives better climb power and I can get almost 5 or 6 minutes more flight time from the same pack.