The measurement refers to the inner dimensions of the furniture socks, i.e., they should be the same as the outer dimensions of the chair legs they are intended to be mounted on. Measure your chair leg/tube, preferably with a caliper, so you get the correct size right away and avoid unnecessary shipping costs.
The height is the same regardless of the size, approximately 60mm.
For hollow steel legs, we always recommend having an inside chair leg caps to protect the felt from sharp edges