If the winds are light, the grass is short, and its not too hot out then a 56 4 c will fly the plane just fine. Now say its been raining and the grass(runway) hasn't been cut in 2 weeks. Its going to take a bit more oomph to pull the plane through the grass up to takeoff speed. Either that or use up the whole runway and hope its up to speed! Another scenario that happens here a lot. You take off and fly around for say 3 minutes. Then suddenly the afternoon winds whip up and now your fighting 15-25 km gusts! The larger prop that the 72 4c can turn now might be the only reason that you can make it back to the runway and land in 1 piece. With out the extra power you can end up watching your plane get tossed backwards making large loops until its out of sight or it crashes. From my experience; going too big creates its own problems. Going a bit bigger is usually an advantage. Especially if the larger engine fits the same size motor mount. I have been running 75 2c engines on 60 size planes with great results.[8D