Basically i cant figure out how to calculate radius of sphere projected onto screen. I use gluProject to get screen coords of projected sphere.

This was recently discussed in the following thread: http://www.gamedev.net/community/forums/topic.asp?topic_id=545204.

The projection of a sphere is not a perfect circle, and therefore does not have a single radius. An approximation should be radius * cot(fov / 2) / Z, where fov is the angle of the field of view, and Z is the z-distance to the sphere. This is in clip-space ([0, 1] on the screen), so it must be multiplied by your screen-size if you want it in pixels.