Hard to give solid advice with so little information.

Best post is no post. A 6" post will 'feel' like it's 3 feet wide if it's in a major line of sight. If you have long-range plans to stay in that house, the cost of professional engineering and structure can be a good investment. Very few situations where you do not have the option of moving support structure- even to the point of adding steel. Don't save a few bucks now and then 'woulda, coulda, shoulda' every day for the rest of your life.

Next thing to consider- two smaller posts flanking that island might look better than one beefy one in the middle, looking like it's waiting for the bus or something...