For resizing the street names shown on the map it's a sys.txt entry:

[interface]
mapfontscale=150 ;100 is default, 200 doubles the default size etc. etc.

There's also these but I'm not sure exactly what they're for, perhaps the roadsigns that appear before an intersection?:

[3d_config]
roadsign_lines_per_screen=20 ; larger the number the smaller the font
roadsign_font_size="8"

Never changed the style of fonts myself but I guess it would just be a matter or replacing the existing ones with the desired ones, renamed to the original name, so long as they're the same format (be it TTF or whatever etc.), hopefully someone will correct me if that's wrong.
In my copy of Primo there's a font folder in data.zip & another font folder in the root directory.
There are some skins that give users the ability to change font styles, Decent Skin is one I think.

As for the font sizes unless there's a sys.txt entry that covers it I think the rest for various displays are changed in the template.css, .ini, .ui and perhaps other files in the various folders (correct resolution, common etc.) within data.zip
Possibly .vpr (polygon co-ordinates) files also depending on what it is you want to change, I've had to change some of them in iGO8 & Primo skins to get the desired results.
If using a skin the ones inside the skin's relevant folders will affect things too.