Well GoldKid, the answer is quite simple here. The LENGTH of your highbanker has nothing to do with how much water flow you need! The part that sets how much water is needed is the WIDTH and what type of riffles you're using! 99.9% of highbankers and sluices are a constant width along the entire length so what's enough water at the top is going to be enough at the bottom end of the run. A good rule of thumb is to try to have the water at least an inch or two over the top of your riffles.
A highbanker or sluice needs to be "Tuned" when you set it up. Usually they're set up for one inch of drop per foot of length. So on a 3 foot box, the bottom should be about three inches lower than the top end and they should be as close to dead level across the width.
Learning to use a highbanker or sluice is not all that hard but you will need to experiment a little to find that perfect angle and water flow combination. Things like riffle height, type of matting and even the materials you're running can change what's needed for catching the most gold. After some time using your gear, you'll know what works best for just about any conditions.
As far as powering a sump/bilge pump goes there are a couple of options. You can use a marine battery (deep cycle type), a generator with some kind of power supply or solar. I've seen and tried about every possible way and each one has both good and bad points to it. You have to allow the dig site determine what equipment is needed. Larger pumps are always a good idea because you can adjust the flow with a simple valve setup. As long as your riffles are clearing out in 12-15 seconds you've got enough flow. If they're not, you're either feeding the material to fast or you don't have enough flow.
You didn't say if you're setting this up as a recirculating system or drawing water from a stream and letting it flow right back into the stream. If you're doing a recirc system you'll need to have some kind of filter around your pump to keep it from clogging up with plant materials.
If you've got any more questions, feel free to PM me and I'll do my best to answer them.
G.I.