I am going to put artificial grass down, and this will be 40mm thick. there will be 50mm of hardcore and 25mm of sharp and on top. That all adds up to 115mm.
Now then, does the wood sit ontop of the hardcore or does go down the side of it. Whichever way it is , that should determine the size of the wood. Googling says to use 3inch x 2inch, but is that deep enough.