If your sink is on the island the number and size of pendants is dictated by the size of the island. Even if the island is on the smaller side (6' or less) I would suggest two small pendants (5-6" diameter) placed on either side and slightly forward of the sink. If the sink is under a window with upper cabinets on either side, for example, then yes one pendant would work. The size of that pendant would be driven by the size of your opening and sink. Hope this helps answer your question.

ok here is a pic...its only one light over th sink and the sink is in the middle of the counter top

size of island? thank you

Here is a similar style pendant light http://www.wegotlites.net/One-Light-Satin-Nickel-Metal-Shade-Down-Mini-Pendant--Savoy-House-7-4131-1-SN_p_149434.html

The island is 8' x 4'