When determining the diameter of a light fixture, always follow the 1/3 rule. The diameter of your pendant light should be no more than 1/3 the length of the kitchen island.
For instance, if your kitchen island is 12 feet long, your pendant light should be no more than 4 feet in diameter. Just remember to choose smaller pendant lights if you plan on hanging them in a group; this way, they will not appear crowded.