[el_zorro]

The diameter actually does have an effect on where geostationary orbit is: it changes how fast the surface is spinning, and hence the orbital speed needed for a stationary orbit.

Mars spins once every 24 hours 37 minutes. If the diameter was twice as large, it would have to be moving twice as fast to maintain that length of day.

Interestingly, since you orbit faster the closer you are to the surface, a decreased diameter actually pushes geosynchronous out away from the planet. This effect is, of course, very much negated by the absence of mass. Still, two equally massive planets with the same rotational period but NOT the same diameter will have different geostationary altitudes.