I imagine the first number on the propeller size is how many inches it is.
What doesthe second number represent??

Yes the first number is the diameter and the second is the pitch. The pitch is the 'inchs of development' with one revolution of the propellor.

In other words it is the distance the propellor would move forward in one rotation if it were operating in a theoretical solid.

John

Second number is pitch, usually in inches. Pitch is the theoretical distance the propeller would travel in one revolution, so a 5 inch pitch would pull the prop 5 inches forward through a theoretical solid. Low pitch gives good initial pull or torque, high pitch gives faster top speed. Check your engine manufacturer's manual for a range of different diameter and pitch sizes that will work well with your engine.

Instructions first, if you look at the top of the header bar there is also a sticky prop chart.

Your starting to ask the good questions Now take what you have learned to the next level. Let's say you have an 11x7 prop spinning 8,000 RPM. In a perfect 100% efficient environment (this utopia does not exist) your plane would be moving at 56,000 inches per minute or 3,360,000 inches per hour or 53.03 miles per hour. Due to drag, vairiable air density, prop efficiency as well as some things I'm not aware of we will never see 100% performance out of our props. Another unknown is how much rpm gain you will have when the engine unloads in the air. Some gas engines will allow you to put a tachometer on the ignition and it will record your peek RPM but even that may not be a good judgement of what your sustained RPM was at any dicearnable single point during the flight. I know you did not ask this question but I find this part of model aviation fun and had nothing better to do than share