the one way i have found how to do this is to open finder from which ever method you want.
for this example let's use the harddisk icon on the desktop. close down all the other finder windows and open the harddisk icon.
now alter that window to exactly how you want it, the sidebar size, window size, window position, and then close it. reopening it should have it exactly how it was when you closed it.
now close that window so that there are no other finder windows open and click the finder icon in the dock, and do the same thing, then close it, and the settings for that finder method should be saved.
works for me.