How to set Finder Window to Default size...

If you mean the size when you first put the OS on, then I believe that's in preferences or you can delete the Finder.plist file.

 

There is already a default size for Finder windows. Once you touch the resize corner, you lose it. All windows that you close will reopen at the same size and position. The problem comes in by two avenues: resizing and opening new folders in the same window. Those things screw everything up.

There is a tedious remedy, if you want all Finder windows to be the same size. You can open the top level folder (#1), then size it as big or as small as you want. Now, open the first enclosed folder in the same window, then close it. Now open #1 again, and open the second enclosed folder.... etc. If any enclosed folder encloses other folders, it's the same drill.

Once you get all folders to be the same, default size, you're done. But, you're undone if you touch that resize corner!