Can't tell you the size of the star. But I will tell you that the screw is more than likely reverse thread. Turn it to the right to loosen it! Then after you remove the screw use a fairly good size (ell) shaped allen wrench. Put the short end in the chuck and close the chuck around it (like it was a drill bit) Then tap on the long end of the allen wrench (hex wrench) with a hammer to remove the chuck from the drill. Just did it to my milwaukee half inch and it worked great. Oh ,soak that screw in wd overnight too! It will help breaking that bugger loose.