Finder window does not remember size. Yosemite
Do a backup in case you mess up. (For experienced users who understand what the next steps do, this can be skipped.)
Open the Finder
Press SHIFT+CMD+G and enter ~/Library/Preferences/. This will open the Preferences folder inside your personal library.
Find the file com.apple.finder.plist and move (drag) it to your desktop.
Re-launch the Finder: Click the Apple icon in the top left corner, select  Force Quit, select Finder and kill it. You can restart, but this is quicker. 
If it works okay, delete the plist from the desktop. If not, return the .plist to where you got it from, overwriting the newer one.


This worked for me.

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