Have fun with the surface area
As far as the wire, you can use whatever size you want as long as it's completely in solution. You'd have to wire everything up, and then connect that to a thicker copper wire, that the thicker stuff is what makes the transition from outside the tank to inside. You don't want the thin stuff out of the solution too. What happens is that in the solution the heat of the current in the small dissipates into the solution, and outside the same effect won't happen with air, so the wire gets hot and can melt.
On all my tanks I make up an S shaped hook out of copper pipe with a small hole in the bottom of it. The hook has to be long enough that the hole in just into the solution, from there you use whatever size wire you have handy. Save on always having to use thick wire, and it saves that the wire you hang with doesn't have to be as long.