Wow....Ok.....I read a confusing article on how to calculate this just now.....there are a bunch of rules and it can depend on the size of the room and the height of the ceiling.

But the rule that made the most sense to me is that you want the 6 inches of space around the perimeter of the island to be free of lighting so that someone sitting down or leaning over the island wouldn't bump their head on a fixture. So taking into account the width of your island which is the smallest measurement, is 40". Then subtract 12" from that and that leaves you a maximum width of 28". That's a BIG pendant IMO. Also if you are using two fixtures, according to the "rules" (I would call them guidelines and you should feel free to break the "rules", you should have the same amount of space between the pendants as they are wide.

So, if you find a 20" pendant then you should have 20" between the two as well.

In that case considering your length, your pendants would hang covering 60 inches of island from the middle. That means you'd have 24" free at either end of the island.

I see that one you are showing on the Kichler site (the Everly is the one I think) and I see one that is 19.75" and one that is 15.5".

I've read others on here saying they found balloons approximately the size of the light fixture and hung them from the ceiling to approximate the size and look. It might be worth a try.

Or worst case scenario, order one of each size and see which one appeals the most (assuming they have a good return policy).

I do like the fixture you've chosen, it looks very nice.

Our ceilings are 10' and our island is 8' long x 4 ' wide. We have 2 Pottery Barn bell-shaped pendants. The widest part of the bell at the bottom is 13." It looks proportional in my opinion. There's more than one width of space between them. We eye-balled it with balloons as suggested above, and then measured carefully to be sure they were equal distance from all the edges. I'm looking at them right now and I think if they were 20" they would be too huge, but I'm no designer.

The inside of my tray is 10', my island is 5x8. Our 2 pendants are approx 15" wide, 19" tall, but they are black. The size looks great, I would not want anything larger.

You have to draw it up and it's likely a range of sizes will work. Here are your 18" diameter fixtures. But you can see that you could go a bit larger if necessary.

thank you so much for your help-exactly what I needed! I've been overwhelmed with all the other design choice for this house build and this helped me visualized--thank you!