a 10X6 is the recommended break in prop for the .40LA. You can run the 9X6 as long as it's not in a heavy plane. I don't think it would have the power to keep a heavy plane in the air. I would recommend a 9X8, 10X6 or 11X4, maybe a 11X5, but nothing longer or steeper pitched.
Starting with the 10X6, if you go up one inch in length, go down 2 in pitch.a MAS 11X5 prop would be stretching it. It would still turn almost optimal RPMs on the ground, but it would lose speed in a sharp turn.
I like the LA engines. I use them in most of my SPADs, but they are a bushed engine, and not as strong as a bearing engine. They'll take tons of abuse and come back wagging their tail.
I have to disagree with OpJose. As far as load on the engine, a 10X7 would be the same as a 11X5, and there is no way a .40LA will spin a 12X4 more than 8,000 rpms. Remember, nitro engines get their power from RPMs. The faster the prop turns, the more power it will keep when turning and going vertical.