Every seat is going to take up about 24″ of space, so you’ll need to multiply the seating you want by that number to get the result. If you want to seat 6 people you’ll need an island that’s 144″ long.
Of course, not all chairs or barstools are 24″. Some are 16″, others are 18″, but we’re talking about the width of a person, shoulder-to-shoulder. And it depends on how big your family is and how free they are with their elbows. Some people will want 30″ of space per seat, just to be on the safe side.