12x12 is the maximum, but you can make bigger tanks by letting the tanks share walls/valves. Each tank is considered separate, but they will equalize to the height of the shared valve.
/rimshotOoooo, I didn't know about sharing walls and joining tanks like that ... Tanks!!! (I had to go there)
/rimshot
To expand on buccaneer's post, the two tanks will 'seek their own level', not necessarily equalize volume. That's a non issue if the tanks are the same size, but may be important if they're not.

This.
This is two 12x12 tanks, that share a single valve each with the small 3x3 between them. Lava is pumped in to the bottom of the small tank. Both large tanks filled to the top.
Just don't remove the valve.Something I just figured out. For some reason, smooth stone is not a valid tank wall, but cobblestone is.
I have a question about resizing tanks. How do you do it? I know that with iron tanks, you can change the size without losing the contents as long as you keep the bottom middle block. But is that true of Xycraft tanks also?