Can I permanently increase the font size in the reader pane of Outlook?

No dialogue box appears

The font size is way too small. I can temporarily increase the font size for each email I read or write. I can't find any setting or instructions online to permanently increase the font size.

I had the same problem. I set the reading pane to the right and hid the navigation pane and the message in the reading pane showed up full size.

The problem is that this workaround doesn't change the core issue which is setting the font. This is my problem, so I can either have e-mails that I can't properly preview, or e-mails that are at a bad zoom level to read easily.

This is core accessibility and MS should have sorted this out or be getting hauled over the coals by governments and accessibility groups.

I found a workaround double-click on a message that will be opened as a "full" window to read the message click on Zoom change the percent to your desired value and click on Remember my preference.
this should change and remember the zoom level for the reading pane

It didn't work for me at first either, but I was RIGHT-clicking, and that just shows a completely different dialog box. Try "normal" clicking (left-click) and see if that works.