Also, see http://elite-dangerous.wikia.com/wiki/Fuel_Scoop for the overview of how fast a fuel scoop will refuel your ship. I use a 5B, it refuels my Asp completely in about one minute. Considering upgrading to a class 6 fuel scoop later.
The speed you are flying has no impact on the scoop rate.
Uhm, I disagree faster speed = greater scoop rate
Uhm, I disagree faster speed = greater scoop rate
I've seen the flow rate change with the throttle, although that could arguably also be just fooling with distance.
Get the biggest scoop you can afford, since they fill up much quicker.