5-3/4 lbs is an awesome weight. Mine was 6 lbs 1 oz.

With a YS, you ought not to go below 20% nitro or performance will suffer.

As a general rule, a higher pitch prop will smooth out the acceleration when throttling up, but with a small engine your choice of prop size is constrained by considerations of braking (larger diameter brakes better) and operating on the engine's power curve (a matter of balancing fuel mix, glow plug and prop load). Those who fly higher pitch props (>8) do so on engines with plenty of power in relation to the model AUW, so they can have a big enough diameter for braking and still load the engine with prop pitch. I would not say the YS 63 has enough excess power on the Javelin to justify trading thrust for pitch.

Anyway, the real test is how does it fly, and the flight of the Javelin/YS63/13x6 prop does fly very nicely.

12x6 prop on OS 70 sounds right to me. The OS 70 does not have the power of a YS 63. The someone you're speaking of tells me his OS 70 is "adequate" power for the Javelin. I would say the YS 63 is also adequate, definitely not excess power for this model.

When you fly your plane, you can try whatever prop sizes you want, which is the best way to convince yourself. My recommendation, like the others, is APC 13x6.