If the virtual camera is moving, the distance from the camera will matter a great deal. If the camera isn't moving, it doesn't matter.
3D space is somewhat relative as mentioned above. Distance really becomes an issue when you start adding lights to your composition. The scale of your object and distance will be affected by the position and intensity of the lights.
If you are not using lights it really just matters what it looks like on screen. If it is "10 feet" or "100 feet" doesn't really matter if you like what you are seeing. My 2 cents and hope this helps!
Also affected by z-space distance is the background movement relative to anything closer to the camera. If the background is farther away from a moving camera, it will move differently than if it was closer. That effect is called PARALLAX and has to do with the relative movement of objects in 3D space as the camera moves. Here's interesting video showing the parallax effect on a layered photo spread out in 3D space.
If you want to fit your 3d object into the scene, also note that objects that are far away are usually more desaturated and maybe blurry, because all the air between your eye and the object is in reality not "100% transparent".
Taking care of these details will make your compositing more realistic.