I wouldn't exceed 6" in diameter. Depends on your ceiling height as well. However, if it is 8ft or so I'd stick with 5 to 6" in diameter depending on your space. I'd select fixtures that work best for the space. If there is not a lot of other lighting in the space then you want open or clear glass lights. If there is decent light from cans or..... and you want down lighting you can select cones or similar in metal fixtures.

You didn’t Indicate the height of your ceiling. With an 8ft ceiling it is a challenge because if you will be restricted to a smaller diameter. I only installed two over my 99” island. I wanted them larger in diameter, but the next size would not accommodate my 8ft ceiling. I did not want to use three, I knew that from the beginning. I wanted the lighting to be uncluttered and open. This is what I did.

What you should do is place the pendants absolutely centered on the island but place them 1/3 of the way in from the seating area. Then place a high hat directly above your sink for good task lighting. That's what I did and it works beautifully. Those sitting at the island get good lighting and you working at the sink get good lighting and no one will ever notice if it's slightly off because no one ever stands exactly dead center looking.

We are building the home right now and we need to decide on the wiring for our design appointment. Let me see if I can find a picture from model