The 1/7-size (plus just a little) tank works great for me. But then it's not exactly that simple.
In order to minimize evaporation, you'll want the water to get to its destination tile as quickly as possible. That means a vertical drop from the tank
You want it to spread as quickly as possible as far as possible. That means a more or less circular growing room. I don't make the tanks circular, but I guess you could.
carve out a tank above your growing room that is 1/7 the size of the growing room, plus some extra tiles.
Channel holes in the floor for one out of every nine tiles, so that each hole has at least two tiles between itself and the next hole and one tile between itself and the wall. This ensures that each hole will have access to the maximum nine tiles for its drop.
Put hatch covers over each hole and hook them all up to the same lever.
Pave every single other tile in the tank and in the path the water follows to get to the tank. This will prevent tower-caps from growing in unwanted places and clogging up your works.
Put at door or a floodgate between the tank and your water source, so that you can seal it once it's full. Attach that to a different lever.
Now fill the tank, preferably with pressurized water so that it fills quickly. Once it's full, close the door or floodgate I mentioned a moment ago.
Now, whenever you're ready, throw the lever that opens those hatch covers.
Swish, down goes your FPS for a few moments! And afterward you should have a room full of 1-depth water with a couple of 2-depth tiles chasing around and mud, mud, mud.
My current fort has three of these stacked on top of each other. Each growing room has around 700 tiles. Each tank has around 120. This totally does not produce enough wood to feed my pearlash and charcoal habits.
I totally didn't know either that they wouldn't grow side-by-side or that you should clear the plants out. I just might do the walking-grid and brush-clearing to try and improve the efficiency of my own tower-cap farming complex. All I had was traffic restrictions of increasing degrees across grids of smaller and smaller sizes.
If you're going to make some kind of no-growth paths, pave them. You'd have to smooth the ground after each dump, but paving lasts forever.