Before Rikyu's time, there where only kobukusa. It is Rikyu's wife, Souon, who is credited for creating the first fukusa, whose measurement she made four times as large as a kobukusa.
Kobukusa is about one fourth of the size of a Fukusa. It is (in the lower level temaes) used to put other items on top of. Either to show that one have a high regard for that particular item (Kazari temae) or to protect the item from the ground (Chabako)
Most guests will have two of these with them when attending a tea gathering.
Other schools use a Dashi-bukusa instead of a kobukusa, in Urasenke a Dashi-bukusa may always replace a kobukusa and is used the same way.