The first thing I would say is that, for the best quality picture, you should set your resolution to match the native resolution of the monitor. If you then find that things are too small, on the resolution tab, click the advanced button and adjust the DPI setting - try it first on 120, and if that doesn't suit, select a custom setting.
For your problem, try this:
Open OE.
Click the Restore button (so that it's NOT maximized)
Drag the non-maximized window to the top left of your screen.
Place the cursor in the bottom-right corner (so that the diagonal double-headed arrow appears) and drag it to the full size of the screen.
Close the window.
You should find that in future it opens to the correct size.