Dexter Rucker is a long-time Mac owner, and knows his way around OS X. But he writes that he cannot seem to get a basic bit of behavior to stick in Yosemite:
"Most of the time the Finder window opens up very small and I have to resize it to make it larger, but sometimes it opens how I last closed it. With past OS X operating systems, it opened how it was left which is rather large. Is there a way to have the window open how it was last closed?"
This has baffled me as well, because I've seen inconsistent behavior, and I did some exploring. In my experience, I can't resize a window and then get the next opened window to work the same way. I checked through forums for the wisdom of crowds, and many people suggested deleting a variety of preference or Finder-configuration files. While that's not a terrible thing to do, it also seems extreme.
In Yosemite, I tested one suggestion after I was unable to set a window's default opening behavior. I created a model window, closed it, and then closed all windows (Command-Option-W, Option plus close button, or hold down Option while selecting File and choose Close All Windows). Then I held down Option and right-clicked the Finder icon in the Dock and selected Relaunch.
Voila! After that point, whenever I resized a Finder window, all subsequent windows would open in the same dimensions, offset slightly in location, including the width of the sidebar. I tried this in the current public El Capitan beta, but had zero luck. Perhaps it will be fixed by release.
What's odd about this solution is that I had just restarted the Mac a few hours before. Relaunching the Finder may clear files that persist across OS X's restart.
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