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.
Read more here:: How to get a new Finder window the size you want