Default Window Size and Positions

Within Windows 7, @Owen's statement is correct (I've tested it). However, if you're wanting to reset to the default window size, then you can edit the registry through these .reg hacks.

They appear to edit these registry settings:

Windows Registry Editor Version 5.00

[-HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\BagMRU]
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\Shell\Bags]

[-HKEY_CURRENT_USER\Software\Microsoft\Windows\ShellNoRoam\Bags]
[-HKEY_CURRENT_USER\Software\Microsoft\Windows\ShellNoRoam\BagMRU]

[-HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\Shell\BagMRU]
[-HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\Shell\Bags]

I'm not sure if there's a way to change the actual default size to a specified pixel ratio, but this appears to be the closest to what you're looking for.

Note: These are for Windows 7 machines, and therefore, I'm not 100% sure if it'll work for a Vista machine, though it's worth giving a shot.

http://www.southbaypc.com/AutoSizer/

This program seems to work. Open any Explorer window, choose it on "Currently open windows" list, press AutoSize, choose "Resize / Position", press "Set Size" and choose your size.

Try resizing that window to your desired size then close it. The next time you open it, it will be of the same size as that of what you just closed.