A viewport has a custom scale you can see it when you click on it and using Property.
So rescale the viewport then change the custom scale value to suit. Play with some standard scales and look at what Custom scale value is, metric makes it easy.
You may also want to pick a point in model space (setq pt (getpoint "\nPick ")) once you have this you can use Zoom C pt scale, this will re center your view, then set your custom scale. A ps Zoom valueXP eg Zoom 0.25XP, sets the scale also.
Lastly it's always a good idea to lock a viewport so you don't accidently zoom inside it.