You match the pitch and diameter to the engine, fuel and desired flying style.

A typical .46 runs well using the 10X7 to 12X4 with 10% to 20% nitro fuel. The 9X7 is a bit light load for a .46 even on low nitro fuel. in the range of the .40 size engines, you trade one inch of dia for one inch of pitch to keep close to the same load on the engine.

Trainers as mentioned don't need speed. Beginners typically need power for takeoff and you get more power at low airspeed with larger diameter and lower pitch. the low pitch also helps with slowing the aircraft down to land.

Most ".40 size to .60 size" trainers (the "60 size will typically say .40 to .60 engines are recommended and they fly fine as trainers with a .46.) the 11X5 is a good choice for the .46 engine You can use a 11X6 with a more powerful .46 such as the O.S. FX series. (The LA will not turn a an 11X6 nearly as well). If using a .40 on a .40 size trainer (almost no one does it any more... but a .40 size trainer flys fine with a .40 or even a .25...) you would want an 11X4 if its a low power engine... and 11X5 if its a good ball bearing one.

These are STARTING recommendations... different flying styles will make different props seem better, because they will give you different performance.