Capella is not one star but a system of four stars. The primary star, Capella Aa, is 12 times the size of the sun and has nearly three times the mass. Capella Ab is nine times the size of the sun and has slightly less mass than Capella Aa.
The Capellan system also has two red dwarf stars: Capella H and Capella L. Capella H has a little more than half the mass of the sun, while Capella L has about 0.19 the mass of the sun. Capella H is a little more than half the size of the sun, while the size of Capella L is unknown as of 2014.