Yeah someone shoot me in the head. I was thinking he was making a submerged unit and that was 1 cell he was giving the plate area for. Had he been doing that my answer was correct. My calculation was for submerged cells. -Sorry!
Pertyfly is correct in his calculation with a series cell unit. That is a much more simple matter to calculate.
You just take the surface area of 1 plate (since a cell is made up of a pair) subtract the inch or so from the top due to water level and then you get your total surface area of 1 cell. Now 6x8 plates with an inch at the top will yield 7x6 usable area actually more like 7x5.5 with the groves in the side and bottom. Or in the rubber gasket model I saw Kevin building... you'd have to take a lot more off. Just measure your USABLE surface area carefully guys. It's important.
So for sake of examples we'll just use what pertyfly was saying. Eco has roughly 6x7 inches of usable surface area per plate. That's 42 square inches.
Eco to answer your question all you need to do to determine your max current is take this 42 and multiply it by how much current you want to push per square inch. A series cell is a VERY SIMPLE calculation.
You don't want to go too high or you will get a lot of heat and probably be capped off by current density anyway. I'd start with Bob's suggestion of 1/2amp per square inch which means you can push 21A through your unit. If you take grooves and spacers into account properly Bob stating more like 15A ...and I agree 100%
Experiment from that point to see if you can go higher and still have high efficiency. OK
You tell us how high you're able to go... don't ask us!