Perhaps the easiest way to put it is the way I understand it.
The second number in the prop size is pitch. Theoretically, the plane will travel that many inches per rotation of the prop. So say if I've got a 10x6 prop, the plane will travel 6 inches per rotation. Of course other factors come into play since your plane is not and never can be aerodynamically perfect.
The length of the blades determines how fast the prop will rotate. Longer blades also have more surface area, and thus more pulling power. That's why the 12x6 blade that BLE mentioned above has more hanging power than the shorter 11x7.

Here's a handy speed calculator I found. Measure the maximum rpm with a tach and then plug it in, along with the prop pitch and it will tell you the maximum theoretical speed (in either MPH or KM/H) that your plane could travel.
http://www.missionwings.com/airspeed.asp