A a lot depends on the plane. A slow, fat airfoil has a lot of drag at high speeds.

Depends on what you are looking for.

Go to higher pitch if you want more speed.
Go to bigger diameter, lower pitch for more pull.

High pitch props tend to have less of a pitch-factor. Will require less rudder input on take-offs.

Try going one step in each direction. I.e from 10-6 try 10-7 and 11-6 or 11-5.

A tach is a good tip. You want to be within the range. For example the best prop on my 91FX gave 12400 rpm on the gound. It would hit close to the max (16k) at full speed. If RPM on the ground is too low, it may not reach the ideal power band. If too high, it will max out the engine in the air and will be hard on the engine.