In our guide to light sizing, we recommend spacing pendants 2 feet apart over an island, so you could go for 3 pendants since your island is 6 feet long.

As for sizing, what we do is take the width of the surface in inches (4 feet = 48 inches), subtract 12 from that number (36) and then use that number as the ideal maximum width/diameter of a light to go over it. The largest of the 3 sizes you mentioned, 19 inches, would probably work best and not look too small for your space.

For hanging height, the starting range is 28 to 34 inches above the island. Since you have 8 foot ceilings, that range should work well for you.

Most pendants come with a few different lengths of downrod that you can mix and match to find the perfect hanging height in your room.

I hope this helps answer your questions!