Charlie was on the money with his explainatioon on prop pitch being traded for prop lenth or visa versa.

         But alas a little more insight.  Your engine given it's size makes a rated HP (horse power) at sea level. at a given air temperature.
It's called density altitude....and as you go up higher in altitude or air temp, your engine makes less power and your prop makes less thrust.  This is because the air is thinner.  That is why someday we'll all have super chargers on our RC engines.  But for now only the big boys do. [X(]
      When someone tells you to choose a prop by "the way you fly"  it's because.......the manufacturer of that engine has set forth what size prop that engine is rated to turn.  We'll say you have 2 choices......10X7 or 11X6.   The both put roughly the same load on the engine....meaning how hard the engine has to work to turn the prop.  But the manufacturer has worked that out for you and has told you that, that is what's best for that engine.  If you want a bigger prop....get a bigger engine.    Now....How do I decide between the 2 choices.   In overly simplifed terms......the 11X6 is a tractor prop.
It will pull more weight (your airplane) but at a slower speed than the 10X7.  The 10X7 will take a little longer to get going, but should give you a higher top speed once it gets moving,  But it won't have the same thrust ooomph (go power) at slow speeds that the 11X6 will have.

          And That ladies and gentlemen.....is why they say.......buy how you fly.  Fast or slow???

For CHOCORROL......your club assumes that because you're higher up....the air is thinner and so you need to push harder on the air.  Some lojic to it.
                                     But as others have said....try both sizes and stick with the one that makes you the happiest....pull or speed???