It works like any window, you resize it using the sidebars.
You can use + and - keys to zoom in and out.
If you want to play in full screen, and pick the resultion op2 plays in.
Set it on desktop > propties, settings tab and pick the resultion for your desktop.
Then run op2.
If your not using our version then you have to set it to 16bit for it to use the desktop resultion.
If your using our version, then 32bit is fine, op2 will run using the desktop resultion.
Have fun.