Thank God for this thread. I thought I was going crazy, as I thought this only happened to me. Searched through the Apple.com forums without finding anyone else with this Finder-problem! Ever since updating to Yosemite, my Finder has been reverting its window size to default at random, with no obvious patterns or reasons. Only relaunching Finder helps (not even restarting). This has not been fixed with 10.10.1. I have had this bug on my iMac, rMBP and Mac mini.
But after reading this thread, I realized that on all my devices, I have set Finder to open new windows at Home. I have changed this to one level up, and Finder has been acting normally ever since. Don't understand why this only happens when I set to Home on new window, though.
And I'm curious to why not so many people seem to have been affected by this. I would think that most people like to have Finder start at Home, where all the folders are...