Hello. I'm a longtime Mac user, but never delved into the apple script world. Finder "right sizing" has been something I've been waiting for a long time. Stumbled upon this thread.
I downloaded the kmmacros file, but it won't open in Script editor. Is there a way to convert to apple script?
Download Keyboard Maestro if you don't already have it. Download and double-click the macro to import it into Keyboard Maestro. You can then see the actions in the macro and copy out the AppleScript.
If you've any interest in automating your macro -- and if you are looking into AppleScript you must have! -- you should take a look at KM. It's much easier to use than AS, integrates well with AS (both in running AS scripts and also being scriptable via AS), and lets you do lots of things that are either very difficult or impossible via AS (eg image detection).
The AppleScript itself won't do you much good. IIRC, "System Events" lost the ability to double-click back in El Capitan. Here's a version of the script anyway, so you can see how to get the coordinates of the column divider that needs double-clicking:
tell application "System Events"
tell application process "Finder"
tell window 1
tell splitter group 1
tell splitter group 1
tell scroll area 1
tell outline 1
tell column 1
set thePos to position
set theSize to size
set theClick to {((item 1 of thePos) + (item 1 of theSize)), ((item 2 of thePos) + 10)}
return theClick
end tell
end tell
end tell
end tell
end tell
end tell
end tell
end tell
...and in the macro those coordinates get returned to KM, which does have the ability to simulate a double-click (and much more besides).