A crude but amazingly effective measure of the steric demands of a ligand is the ligand's cone angle. The concept is rather simple: one physically measures the "cone" swept out by the ligand at the metal center. The wider the angle of the cone, the greater the steric influence of the ligand. A cone angle of 180 degrees would indicate that the ligand effectively protects (or covers) one half of the coordination sphere of the metal complex:
For your visualization convenience, some ball and stick models of phosphine complexes along with a cyclopentadienyl complex are presented. These are not presented in space-filling form so you will be able to understand how each component of the ligand fills space or blocks coordination sites:
