Islands can serve as tables for informal meals. If it's the height of conventional counters—36 inches—you'll need stools and an ample overhang of about 12 inches to comfortably accommodate sitters' knees.
If you're thinking about placing a table in your kitchen, here are some basic parameters: A rectangular table with a seating capacity of four to six should measure 2½ feet by 5 to 5½ feet. You'll need 2½ to 3 feet of clearance all around for chairs and adequate circulation. A round table takes up less space but can accommodate more people if need be. Remember that a small increase in radius makes for a big increase in the circumference of the table and therefore the floor space it will take up.