10x7

10= length

7= pitch, or how many inches forward it will go in one revolution (Theoretically)


It's best to follow the recommended size range for your engine. From there you can change depending on your needs.

For example, if your engine is designed for a 10x7 but it won't slow down for landing (And it's not nose heavy which will cause excessive landing speed) you can switch to an 11x6, or even a 10x6, or maybe you want to try 3D and need a lot of pull in which case you could try a 12x3 or 4

Basic rule is for every number you increase in length, you need to decrease one in pitch