I’ve cooked whole fish, fillets (skin on or off), and big pieces of fish, all worked great. Easy way is to rub the fish with a little lemon or lime, olive oil, and your favorite spice blend. Lay it on the soaked plank and grill over medium heat until flaky, don’t have to turn it or anything.
If you’ve got a fish big enough to cut into roughly 1"x1"x1" or bigger cubes, try wrapping pieces of bacon around those bad boys and cooking them on the plank until the bacon just gets a little crisp around the edges… it will change your life!
I prefer oak planks over cedar, harder to find but gives a milder, smokier flavor - I find the cedar to be a bit too “piney” for my taste. It does go well with salmon though!