The first number is the length of the prop, tip to tip. The second number is the pitch. This number is the number of inches the prop will THEORETICALLY move through a SOLID material in one revolution. The capitalized words are important. Because air is compressible, and not solid, there's a lot of slippage so the REAL amount of movement is never the real pitch number.

Now, what does this second number really mean?

GIVEN THE SAME RPM:
A large diameter/low number pitch prop will give more pull and less top speed.
A small diameter/high number pitch prop will give more speed and less pull.

It's like low gear/high gear in a car.

Since you want a trainer to not be too fast, try a 11 x 4 on that LA.

Dr.1