You cannot increase the size of the global font size for project pages. And while you can increase the font size manually within some of the tools, widgets, the size of the headings for the widgets, etc. will remain the default size.
You should be able to change the size of the font for project pages in the Global settings.
Nicola .gillette from the Contra Costa County CA, Health Services Department has requested the same
https://app.intercom.com/a/inbox/htcgh62r/inbox/admin/6781601/conversation/63510500270015?view=List
City of Somerville have asked to change the font size for different articles in the newsfeed too
Hillsboro OR has requested to be able to modify font size for project pages and tools.