In the game, press escape, and go to "game settings". There are options there for battle frequency.
You cannot force battles all the time, as far as I know - normally, 1-on-1 battles are displayed on the main map despite your settings. But the settings will make it always use full-screen battles whenever both sides are armed, and at least one side has a fleet (a fleet attacking a freighter will never give a full-screen battle).
