This may sound dumb, but basically pendents need to be high enough so you don't hit your head on them. Cabinet bases with granite tops are about 36". If your height is 6', that would indicate a drop height to allow for 36" from the island counter top, more or less.

Counter lighting is usually installed on the bottom of the upper cabinets. I strongly suggest Zenon or LED under cabinet lighting to reduce the heat factor.

Here is what I found on one website; hope it helps you.

The general rule for the height of a pendant light is between 30' to 40' above the surface of the peninsula or island. However, kitchen islands and peninsulas are highly interactive areas where family and guests will gather. You'll have to consider the heights of people in your household and their interactions with others who are across from them at your kitchen peninsula or island

Thanks. Since posting, we've decided to put off installing pendants for a while... didn't realize they could be fitted into the new cans, after the fact. This will give us a good chance to decide on length.