Make gameobject size always be the same

I have a 3d world, perspective camera, moving character and one gameobject, which I want to seem always the equal size on screen(for example, half of game screen size). If person is too far from that go, go's size should be larger, if too near, it's size should be smaller, so player will see it exactly the same size. void Update { //how to adjust it's size? }