There are no settings within the application to allow you to change the font size of the ruler legend. We do have this option for measurements, so I will pass this request to the development team for consideration for inclusion in a future release.
I want to update the need for the written improvement: It would be very helpful if the font size of the ruler legend is adjustable. I'm working on a high resolution Screen - and it's really a small font size!
I can actually see the font OK but for quick documentation I often use images of measurements to illustrate results. To use created images meaningfully the axis font is always too small
One solution I found is minimizing Picolog's window before saving the chart. The smaller the window the bigger the text font in your chart axis and comments. I hope this helps!!!