Mac, eh? Try this in your config file

output=openglnb
aspect=true
fullresolution=0x0

Thanks! That worked.

Let's say I wanted to force the game into 16x10 (my screen's aspect ratio), how would I do that? My resolution is 1280x800

aspect=false would do the fitting, but I don't like that personally since it makes games appear undesirably stretched. Remember that DOS machines in their wacky resolutions, be it 640x200, 360x480, always outputted to a 4:3 aspect (mostly)

That didn't work:

I'm using
output=openglnb
aspect=false
fullresolution=0x0

and everything looks like it did originally: in widescreen, with a black border around the whole screen, except now the pixels aren't defined as well, everything is looking blurry.