IE doesn't have a default. It remembers the size of the last window closed. Which is really STUPID!
Watch. Press Help and close the Help window. Now create a new window. The new window is the size as the Help window was. I mean, WHY is their help in the browser anyway? Apple's help is HTML. Why not use THAT?
Also, popup ads are horrible with this. I constantly get new windows opening really small and oddly sized because of the last popup ad.