Go experiment! On my Balsa USA Stick 40 (world's best trainer) I did the conventional thing - .40 engine equals a 10x6 prop. Huh...let's try a 10x7! It flew faster - but that wasn't the hot ticket with this plane. 10x5 flew slower, but the engine was a little harder to tweak. 11x5 - insta-takeoffs! 11x4 it would hover - for a while. This was a weak old Enya .40.

My Goldberg cub. Same thing .40 with a 10x6. I forget what the issue was, but I first went to a 11x5, then a 12x4! Flew magnificently. My brother flew his on a KB .61 with a 13x5.

Wide chord props, when looking from the front, are 'thicker'. In other words, the prop, while having the same diameter and pitch, have a bigger blade. They load the engine more. This is where you start getting tweaky. My old-timer (72" wingspan with a 4-stroke .21) was at its happies with an old Top Flite Super M prop - wide chord.