There is a good thread on the forum about fonts here: http://smf.cataclysmdda.com/index.php?topic=269.msg34417#msg34417
My font preference: Vera Sans Mono 10pt. Don’t bother with making it perfectly square.
Longer explanation:
I think most people are on Windows and use the SDL version of the game. I’m on a Mac, and I prefer using the non-SDL version, though I’ve played around with both. I think SDL might be a little faster for me, but it doesn’t anti-alias fonts at all and shows up darker on my screen. So I don’t have a recommendation on a terminal app for linux (I use iTerm on Mac), but I really like the VeraMono font from the thread I mentioned above. I use 10pt Bitstream Vera Sans Mono. It’s not square, but the issue is that your character’s viewing range is big (like 103 characters wide during the day), and it’s hard to read any font below 10pt or so (you may be able to get some to 8pt, but then it’s hard to distinguish small things like what type of tires a car has). If I leave my font taller than it is wide, I can see the whole horizontal viewing area without scrolling. If I make it square, it doesn’t improve my vision vertically and just handicaps me horizontally. You can still use the compass in the corner of the screen to detect monsters outside of your screen but within the vision of your character, but it’s not as good as just seeing them on screen.