Perspective projection will already make objects "twice as far" look "half as big". You need to one-up that scaling to make sure objects are dot-sized at the FARPLANE. You need two scale factors for each mesh. One of the factors, MinSize, will be shared by all of them and you can just experiment to find it. Place a mesh close to the farplane, then shrink it until it is "dot-sized". The other factor, MaxSize, is per-mesh, giving each mass their own dimensions.
A simplified example:
To be dot-sized at the FARPLANE, a MinSize of 0.01f looks good.
To be planet-sized at the NEARPLANE, a MaxSize of 50.0f looks good.
percentOfFrustumDepth = distanceFromCamera / FARPLANE;
//Using linear fall-off
//currentPlanetScale = lerp(MaxSize, MinSize, percentOfFrustumDepth);
currentPlanetScale = lerp(50.0f, 0.01f, percentOfFrustumDepth);
This makes the planet scale MaxSize (50.0f) at the NEARPLANE and MinSize (0.01f) at the FARPLANE. At 50% scene-depth, planet scale would be about 25.0005f.
You can adjust the fall-off any way you'd like, clamping it between X and Y, using different factors between DepthX and DepthY, etc., etc.
//Quadratic(?) fall-off
currentPlanetScale = lerp(MaxSize, MinSize, sqrt(percentOfFrustumDepth));
With this formula, at 50% scene-depth, the planet is about 1/4 of its' full size (71% between Max and Min). Around 25% scene-depth, the planet is half-sized.
//Don't lerp objects closer than 200-units
percentOfFrustumDepth = (distanceFromCamera - 200.0f) / (FARPLANE - 200.0f);
//(distanceFromCamera - 200.0f) might be negative so clamp
percentOfFrustumDepth = fmaxf(0.0f, percentOfFrustumDepth);
Experiment. To give a warp tunnel effect, you can also collapse the object's translations toward a single point, based on their scene-depth.
currentPlanetLocation = lerp(actualPlanetLocation, vanishingPoint, percentOfFrustumDepth);
With this formula, all incoming objects appear to originate from the same dot, spreading out to their actual positions as you get near. All out-going objects appear to collapse into the same dot, then vanish. To see what these two scaling and perspective effects might look like together, watch a few videos of ships warping in EVE Online.
A closeup of the "3D" partition:
