Is there a way to change unit size of a game I already started?

It was fine before, but now that I'm getting into big battles, my computer can't really handle it.

No, you must restart the campaign.

You should be able to do it with the MedManager utility, which allows changing the following in the middle of a campaign:

o Campaign difficulty.
o Battle difficulty.
o Unit scale.
o The current season and year.
o The season and year the campaign starts and ends.
o The number of years per turn.
o Whether dates or turns are displayed.

I haven't used it in a long time, but it worked fine the one time I did try it. Here's the main page for downloading it:

http://www.twcenter.net/forums/showthread.php?t=68328

Apparently it's a little funky working with version 1.2 save files -- see the info here:

http://www.twcenter.net/forums/showp...&postcount=104

As always when working with a new program, make a backup copy of any critical save game files before messing with them.