I asked "Undertow Support" the same question; this was their reply:
There is no font size control in the program. However, the screen font is greatly influenced by the system resolution.
I am using a hand held magnifying glass and still can't find the towns and cities in a reasonable time frame. My screen resolution is 1920 X 1200 on a 15" laptop screen which really squeezes fonts down a lot. Any other resolution setting just makes the usable portion of the screen smaller.