After installing the game it played nicely in full screen mode. Now it will only start with a very small window. How to get it back to full screen mode?

Kind of late but if you still need something for this:

Not sure what happened but you could try ALT + ENTER while the game is open. This shortcut is normally available to make games go to window or go to fullscreen.

There's other things that could have happened here but ^ is generally the first thing to try.