Quickly getting a larger text size can be very useful to people with visual difficulties of any kind, and is a highly recommended feature on pages like legacy donation pages and others where an older viewership is expected.
“[…] it would be great to have a text size widget built into all webpages, accomplishing this task would not only improve the legacy audience experience but would advance our organizational Diversity & Equity goals.” – Jackie Gallagher
Perhaps a page setting to enable a larger-text icon on the pages where this is relevant?
== UPDATE 17/11/2020==
The P4 team decided to decline this idea as users are nowadays pretty used to zoom (on mobiles) and change browser font sizes (on other devices). Plus, all pre-defined sizes and accessibility changes now respect the highest accessibility standards.