Try this:-
- Click on a link to open a new (half-size) screen.
- Close all browser windows other than this new one.
- Drag it into the place that you'd like it to be (eg all the way up into the top left hand corner of the screen), and then drag the bottom right hand corner of the window out as far as you want it (into the bottom right hand corner of the screen if you like).
- Then close the window.
Hopefully this will reset the stored "preferred" size of the window the next time you open your browser.