To change the Solar-Putty font size follow these steps:

1. Open the Solar-Putty SSH client (for example by opening the console of a GNS3 device)
2. Click on three dots in the upper left corner
3. Select Settings,
4. Go to the bottom menu item „GENERAL„
5. Look at the bottom of the window for „FONTS & COLORS“
6. Click on the „Launch Putty“ link. As the result the Putty configuration window will be opened
7. Click on the Default settings inside the Saved sessions
8. At the left menu select Window => Appearance
9. Click on the Change button and set your preferred font and its size. When you will finish click on Ok.
10. At the left menu select „Sessions“ and „Default settings“ again
11. Click on Save
12. You may close the window now. It is done!

Here are screenshots: