I liked what carbine said about the paddle being like an anchor..Imagine paddling through firm mud..You stick the blade in and pull/push yourself along..It's not like sweeping a broom....But IMO blade size can give more power..I've tried different blade sizes and I can overpower small blades but I think paddling with a smaller blade helps develop a better technique.
I looked into correct technique a while back..because I didn't want to develop bad habits..and just wanted to know what's right.
I'm no expert but here's what I think.
You need to paddle with pretty much straight arms through the power stoke..and the power is more from pushing with the top hand than pulling with the lower.
Keeping your back straight and your lower arm almost locked straight it swings through an arc..also with the paddle loose against the fingers. (no need to squeeze)
The top arm is also almost straight as you push the top end of the paddle..Keeping arms straight uses more of the bigger muscle groups..Try to relax the fingers here also.
You reach with the start of the stroke and stop when your lower hand meets your thigh/hips area..or when your arm is vertical.
Right at the point where you stop the stroke and lift the paddle..if with your top hand you twist your thumb ever so slightly towards yourself..you will feel the paddle almost jump out of the water..I think that's what feathering is.
You can bend the arms when bringing the paddle forward but straight arms when stoking..Also when paddling on the right side your left hand should be over the right rail..This feels a little twisted at first..but doing this and paddling deep and close to the board you will go straight..and not need to sink a rail much to hold a straight line.
Also regarding power..Paddle length is important..You want to not bend over too much..Bending over in the surf is ok because you don't paddle for that long without waiting for a wave or turning on or off one..But paddling when not in the waves you don't want to be bending over too much..so..you need a paddle length that lets you remain upright and at mid stroke your top hand should be pretty much in front of your face or shoulders.
Imagine trying to push a post over..You would push with your arms almost straight and you would push not too high and not too low..about shoulder to face level.
Here is a few stills from a vid that I took of myself a while ago on my old NSP board..btw my paddle is a little shorter now..and even back then I had been sup'ing for about two years.
I must do it again with my new Naish board and from the front view showing how the left hand is over the right rail..and vis-versa.
DJ