It doesn't really depend on the layout, especially when it's small like yours. It depends on the load.
How many locomotives will be running simultaneously? How big is each? Are they to be pulling any lighted cars? A smoking caboose? Freight cars with motorized (track powered operating) stuff in them? RailSounds diner or operating car?
Are you relying on the rails to carry all the current, or will you have jumper feed wires to track blocks to maintain even voltage around the layout?