Well, you could scale the png file itself in GIMP or some other bitmap editor and reimport it - ready scaled to be the right size once it's imported.

I really do recommend you opt for the temporary pain of moving your symbol folders and your catalogue files into the Symbols folder and using relative references as you repair the catalogues and map files. Once you've done it on one machine you can copy the resulting folders and catalogues to the other machine and put them in the same place in that Symbols folder. That way, you won't ever have to carry on remaking catalogues every time you sit at the other machine. That would be a pain that will never go away.