The default HTML renderer in current versions is IERenderer which offers configuration settings via its own dialog accessible by its button on Pegasus Mail's main toolbar (usually the rightmost one). The version coming with PM 4.73 provides an option for selecting different font faces and another one for increasing font sizes like browsers do (magnifying pages as a whole). Here's a screenshot of how to access it.

I recently acquired my first 4K resolution monitor.  For some apps it's great; for others (including Pegasus), I can set a property of the .exe not to scale fonts.  (Before I did this I could read everything, but the scaling was such that things were quite blurry).  I then have set the specific fonts I want where I can (e.g., the message body; can't seem to change the text on buttons, some other minor locations).  It's great when apps handle the high resolution well - I find reduced eyestrain, and overall just better aesthetics, so I really don't want to go back.

The big remaining problem is reading HTML messages; the font shows up as so microscopic as to be unreadable.  I'm not happy HTML is used in messages, but at this point one needs to live with it.  Looking at the occasional HTML it doesn't look like a specific font size is specified - so, if possible, I'd like to change the default font for HTML rendering.  I found nothing in the FAQs, but did run across something called BearHTML, which apparently Pegasus uses to render HTML (I say 'apparently' because for all I know this has changed - there seems to be no way to dig 'under the hood' to verify this).  I've done a registry search for BearHTML, hoping perhaps there'd be a registry key to change, but nothing turned up.  

 FYI I'm running Pegasus v4.73 on a Windows 8.1 machine.

I would think this problem/issue will become more common as 4K monitors proliferate, so it will eventually likely have to be solved.  If there's some simple stopgap measure (e.g., changing settings in BearHTML) I'd love to hear it.