Dear ArielBalter,
Good day!
Thank you for posting to Microsoft Community. We are glad to assist.
As per your mentioned description about "Can I permanently increase the font size in the reader pane of Outlook?". After thorough research and testing, I found that increasing the size of the text in the Reading Pane only works on the email you are currently looking at. When you click on another email, the body text will return to the default font size. For extra reference kindly refer to Change the font or font size in the message list - Microsoft Support

It has been concluded that to increase the font size in reading pane permanently not yet available. However, thank you so much for bringing this to our attention, your suggestion is very helpful, and I believe it could make the use of Outlook even better. We will surely raise your concerns to our Product Development Team and hopefully this can be included in the future updates. In the meantime, please provide feedback on this feature request by creating a new post in the Feedback Portal at https://feedbackportal.microsoft.com/feedback, Microsoft is committed to enhancing the customer experience by continually examining suggestions to include in the design and development of future product upgrades based on your suggestions. Hopefully your suggestion will help many other people in the future. Customers’ feedbacks are always valuable for us. Your time and understanding will be greatly appreciated.
Thank you for your time and understanding. For other questions, feel free to add your post in the Microsoft Community. Have a nice day!
Best regards,
Inema | Microsoft Community Moderator