How does the large and huge unit size switch work? Sorry if these are elementary questions but I'm new to RTW and to the TW series, and I wasn't able to find similar information in the forums here.

Right now I'm playing through my first campaign, and the only significant issue I'm having since patch 1.2 is the inability to move large populations around. Can I switch the large/huge on just for particular units or is it all or nothing? Can I switch it mid-campaign? Is the default for the Roman campaign already large, with peasants and some barbarian mercs at 120 men?