Unfortunately this is a problem with hi-resolution monitors displaying the fixed font sizes used in much of Pegasus Mail.  This has been discussed a log with no ideal solution being identified.  Some folks use screen magnifiers, other (like me) reduce the resolution of the monitor.  The optimal resolution for my laptop is 1920x1080 but I run it at 1600x900. 

You may have already looked at the help file but if you haven't, press F1 to open it up then in the Contents tab scroll down to fonts to see info about the fonts that can be configured.

I have now downloaded and installed Pegasus, but tried changing the font size in the interface, message pane, etc. but found that it changed some messages to the larger sizes I choose but not in other which remains very small. I assume this is due to my version of windows. Is there a way around this at all?