I would have to make an educated guess...and say to match the amperage of what ever valves you are using, since basically the switch is what actuates your valves. The manufacturer of the valves should have that information available.
rewire the valves to where the switch is the ground and you can run any kind of switch you want with no worries.
The switch would still need to be rated for the correct amperage whether you are running them off the positive or negative. But there isn't much to worry about with valves. They draw nearly nothing.