Xcode can set text sizes quite flexibly...
Xcode > Preferences > Themes
Source Editor
Console
...but as you have seen, this seems to have no effect on the Developer Documentation
But there's also:
Xcode > Preferences > General
Navigator Size: {System, Small, Medium, Large}
...which changes the size of text in the Navigator (the "side bar", both in Xcode itself, and the Developer Documentation)
...so this is one of the things you wanted.
A small win?
Topic:
Developer Tools & Services
SubTopic:
Xcode
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