KSP: How to move parts closer/farther in the editor?
Solution 1:
Holding alt will disable "surface attachment" - meaning parts snap connect only at connection points (the green spheres).
Other tips:
Don't zoom out too far.
If you lose a part, just grab another. Parts that aren't attached will be removed on launch or open.
Save large frequently used parts (like main engine blocks) as sub-assemblies.
Undo with ctrl+z if you really mess up.
Solution 2:
Although your probably looking fro a proper answer, you can't!
However, once you've placed it, you can then translate it along the XYZ axis.