Propellors make my head spin too

I've been trying to get my head around the black arts of propellor sizes. It's tough. Our old one was 19 x 16 which means 19in diameter and 16 somethings (I don't know what) in pitch - that's the twist in the blade that creates the screw action.
Now that, according to prop experts Crowther, was too big - it should be 19 x 12. And now it is after a Streethay prop-swap.
Apparently the bigger pitch was too much for our engine to push when asked to work hard so it wouldn't reach its higher revs needed when, say, pushing against the flow of a river. The reduced pitch means it can - but on the other hand it also has to rev faster when cruising at lower revs on a canal.
Got it? I'm not sure I did until I compared it to bicycle gears. You need a high top gear for good maximum speed but if it's too high your legs won't be strong enough to turn the pedals round and reach that top speed. So make the gears a bit lower - your legs spin faster, true, but you can make them work better. It's all to do with power and torque if I recall my school physics.
Anyway, after feeling our way for the first day we are now pretty happy with our new prop.