If you open the config file you will find two lines:
fontsize 0.9
fontcolor 255,255,255
Reducing the value of fontsize will make the text smaller. Try 0.1 or 0.2. The fontcolor is the colour of the text which in this case unfortunately is white on white. I've got two solutions. Firstly make the text black by changing the values to 0,0,0. Secondly you could make the board a different colour by filling in the white background on the texture sigboxsides.tga with the colour you want. Unfortunately this also makes the back wall of the inside of the box the same colour. I have only tested this on Local Signal Box so the second solution may be different for the other boxes.
For Signal Box the same method works. A size of 0.2 seems best. Note also the fontcolor is 0,0,0 (black). you may want to change it to 255,255,255 (white) if you change the background colour.
For Signal box (ground) and (platform) unfortunately the board has the same colour as the window frames. As these are textured any simple attempt to replace the colour loses the texture so you must make your own choice.
I hope this helps.