Here's a tip that might help for some users:
I've been working on this problem for an elderly relative with poor vision, who has used pmail happily for many years and doesn't want to learn a new e-mail program, but struggles with the small fonts on a large monitor.
Hopefully Pegasus Mail version 5 will replace hard-coded bitmapped font sizes with scaleable fonts. In the meantime, if you want to change the display DPI for Pegasus Mail but not for other apps or system displays, you *can* do that in Linux (running pmail in wine) but not, so far as i know, in Windows.
In Linux, run the wine configuration tool from the application menu or by executing "winecfg" from a command line. You can go directly to the "graphics" tap, and set the emulated DPI for all applications run in wine, including pmail (e.g. changing from 96 to 144 dpi in winecfg will make all the hardcoded fonts in pmail and all other wine applications 50% larger) . Thta may be easiest of pmail is the only application you run in wine.
If you use other applications in wine, you can set the emulated DPI specifically for pmail by going to the "applications" tab in winecfg and clicking on "add application". The set the graphics emulated DPI for pmail only.
Of course you can change the screen resolation in Windows to make pmail more accessible to a user with poor vision. But so far as I can tell, you can't do that just for one application the way you can with an application runing in wine on Linux.
A user with poor vision may not want to switch to Linux and learn a new OS, but it is a possibility to consider if pmail is a priority.