Change Finder window text size via AppleScript or shell?
Ze'ev, amigo:

This sounds like a job for the shell, and we might get you the functionality you desire in possibly two ways.

First, let's try the elegant method: Without closing any windows, send a SIGHUP to the Finder process, shell-side, after you change that parameter in the AppleScript.   Specifically, add the line: 

do shell script "pkill -1 -l Finder"


The second argument to pkill -- the '-l' -- is for debugging purposes.  If this works, remove the '-l'.  

On a side-note, the font size change is also something that can be changed from the shell-side, via the 'defaults' command.

Please let me know if that works out for you.

Best of luck,

F.