This article will attempt to help in your decision about what size of a pendant to use over your kitchen island, and how many pendants to use over your island.
First the recommendation is The Rule of Three. This rule emphasizes balance by repeating an element three times. Above an island, three pendant lights create a more balanced space than two or four, though the proportion could change the rule of three. In a kitchen with a small island and oversize pendant lights, two pendants would provide enough illumination and three would overwhelm the room. Usually 3 mini pendants are enough for an island that is 8 to 10 feet long. Approximately 2.5 foot apart in this case. But, trust your eye, no one likes the look of pendants that aren't centered above an island. If 4 mini pendants are chosen for one of these bigger islands then 2 foot apart is a better option.
No need to be all matchy-matchy with your pendants with the other lights in the room. The same color family would be okay, mixing the different types of glass might not work out so well. For example: if you have an open concept kitchen with an amber glass chandelier over your nook or dining room area, a clear glass pendant over the kitchen island might not work. An open frame pendant, or an amber glass pendant would work better.
So you ask how high should I hang my pendants from the island top? Well usually they hang 30 to 36" from the counter top. But when deciding between those 6 inches, consider if there is a cook top in the island, if the cook is a taller person. Usually pendants are hung above eye level of the main cook in the house. So that when that person is cooking the pendants won't block the view to the rest of the space for the cook.
Also to consider, what if someone is sitting at your island and the light is too bright for their eyes? It would be nice to be able to dim the lights just a little to ease any discomfort your guests might experience.
In the end it's all about balance and the amount of light in the room. Mini pendants are easy to place and come in a variety of styles. While large pendants could be harder to figure out and sometimes have more light branches and would provide a lot of light. Also to consider is ceiling height the shorter ceilings the smaller the light should be, a large light could overwhelm a room with an average 7' 10" ceiling. Again make sure if your going for large lighting with lots of branches of light, put a dimmer on them. You won't be sorry. Also consider some ambient lighting added to your kitchen. This could be achieved with under cabinet lighting and adding some can lights around the perimeter of the room.
So in review in my experience;
Smaller islands 4 to 5 ft. long can handle 2 (large to medium pendants)
Medium islands 6 to 8 ft. long can handle 2 large pendants or 3 medium pendants.
Larger islands 8 to 10 ft. long can handle 2 to 3 large pendants or 3 to 4 medium or mini pendants.