Will depend on the weight of your hull. I have a Tracker 1648MV with an older MotorGuide 12/24V. At 24V it puts out 41#'s of thrust. Works well for me fishing the local river. Can it up current good enough and hold in any current I've encountered so far, although a 55#, or bigger, would be a nice upgrade. Not enough of an issue for me to invest in a bigger one though. My hull is very light though. Probably only .080 gauge, if even that. I ran it on 12V for the past 2 years. Finally rewired it this spring to get 24V and the full 41#'s of thrust. BIG difference, obviously.
With all of that said, I would get the biggest you are willing to pay for and/or have the batteries to power. You can always run it on a lower setting but would have the extra power when you need it. I'm running mine on full blast to get up current most of the time. Running it on a lower setting will save battery juice and probably be less hard on the motor in the long term.