We got the 4-inch ones which made the window bench plenty tall. It’s the perfect height for kids to be able to climb up on the bench, and for adults to sit on it. We got 3 sets of Capita legs because we made two window benches.
They are easy to screw into the bottom of the Ikea Expedit shelf. We put the legs on each end and in the middle of the bench, totaling 6 legs.