In order for the satellite to remain above the same point on Earth, the rotational velocity of the satellite must be equal to that of the Earth. Knowing the angular velocity, you can use two equations, the gravitational force and the centripital force, setting them equal to each other to find the radius.
Remeber also that by setting the equations equal to each other, you can get rid of the mass of the satellite. It shouldn't matter in the calculation of the altitude. Satellites of any mass should stay in geostationary orbit at this altitude.