IMo the size needs to light the space and usually the size that works with the island zMore important is the actul light bulbs you choose since a walk in closet lighting should be as close to natural daylight as possible so a fiture that is LED 4000k bulbs id the first item since the island is so naroow I woud porobably look at linear fixtures almost as long as the island wouldbe very cool. Or go big in proportion to the space after the storage is decided. Make sure there are no shadows anywhere too .