I presume you’re on Windows, because the Linux/OS X versions should size themselves to your terminal window.
You’ll want to tweak the viewport width and viewport height settings, which control how many tiles the main window uses (from what I understand). Separately, there’s also a FONTDATA file in the data/ directory, which controls how large each individual tile/character is. You may have to play around a little with both of those until you’re satisfied.
Hope that helps :).