Measure the kitchen floor space to define the island. Keep in mind that you will need at least 12 in. for the island overhang. The length and width of the island will come into play as well. Let's say for example, that you want to construct a rectangular-shaped island. You will need to make the island top approximately 6 ft. long and 4 ft. wide, minimum, in order to place three stools on each side.
Envision different seating arrangements. Let's say that you will seat three people on each long side of a rectangular island. Each person will need 12 in. of depth for their knees and a plate of food--which is the overhang area. Each person will require at least 24 in. of width along the bar. This 24 in. width will keep diners' elbows from clashing during a meal. Space in the middle of the bar will hold food, bowls and wine bottles, perhaps. You can build a square bar, of course, if you allow room for the overhangs and 24 in. of seating width for each person.
Allow sufficient room between the bar and other nearby counter tops. While 3 ft. of clearance will work reasonably well, it's better to allow 48 in. between the bar and any nearby counter top. Create the bar to have room for diners to scoot out stools and navigate around the kitchen.. Six people need more room to maneuver than do four.
Use cardboard to represent the layout in an existing kitchen. If you will build the island in an existing kitchen, obtain large pieces of cardboard to cut as templates. You don't want to begin the project of construction until you know you have room to seat six people. There are various ways to remove a base cabinet or switch appliances around to make more room.
Create the ideal bar island size in the planning stages. A square bar that will hold two chairs on each of three sides can work. A 5-ft.-square bar top will work for this situation. If you want to install a design such as a rectangular bar with six chairs all on one side of the bar, think in terms of overhang space and adequate serving space. For example, a bar 3 ft. wide and 12 ft. long is workable for this scenario, allowing for a 24-inch wide sitting space for each diner along the length of the bar. Create this type of island bar only in a very large kitchen, however.