I am visually;;y impaired. How do I increase the size or boldness of the type?

in addition to checking the locations specified by tbirdvet.

you can go into Apple menu > Mail > Settings in the "Fonts & Colors" tab and make changes accordingly.

Open preferences and look at the following:

Display, pick scaled and change to a larger resolution

General, and pick dark mode

Accessibility, and pick increase contrast

All of these should help

You may want to read some of the suggestions in this thread by Old Toad.

How can I permanently change the default … - Apple Community

just to add, a lot of other apps have adjustable font sizes in their settings as well.

for example "Text Edit"