Object rendering at constant size from camera view

Hi !
I’m developing a 3D tool, and I want that my axis object (like in all 3D modelers) always have the same size from camera point of view. Is there an easy way to do this ?
The object must keep his original transformations, so I think I need to draw it as his real position and just resize him (the 3 axes) depending on the distance between the camera and the axis.
But I can’t find the right way.
Thanks in advance for your advice.

Scale the object by distance to camera.