Hi!
What prop size you choose depends on what plane you intend to use the engine in.
10x6 (RAM or Graupner G-Sonic is the best) is for low winged pattern type planes.
9x6, 9x6,5 or 9x7 (APC) is for Q-500 type planes (130cm in span weighting 1,6-1,8kg).
11x4 is for fun fly type planes!

The Rossi .40 is/was as standard equipped with a head shim. I would use no more than 5% nitro and a Rossi 3 or 4 plug. (Nova Rossi 4 or 4A is also good chosies).
Enya 3 or OS 8 will work good too.