First of all, if you directly edited the video8.cfg file to plug in a resolution, then you need to go back and undo that. There is an option in the "Video Settings" whih shows all resolutions your video card supports.
If you plug in a resolution which is not supported by the video card, but happens to work, you risk damage to the video card, and/or the monitor.
The game scales the radio text in an attempt to keep it approximately the same size, regardless of the resolution. The higher the resolution, the higher the bit density of the text. There is no config option for this.
The font given to the game comes from Windows. If Windows cannot give the game exactly what it wants, then Windows approximates the selection.