Shortcuts allows changing the system's text size to specific sizes, but there's is no easy way to tell Siri to make the text larger or smaller than the current size.

The Text Size shortcut is the first ingredient to solve that.

This shortcut is part of the second ingredient: a pair or shortcuts that change the relative text size:

• Larger Text [DJ] (this shortcut)
• Smaller Text [DJ]

These two remember the current size (changed by the "Text Size" shortcut) using Data Jar. There is also available an equivalent pair of shortcuts that do the remembering using a file in iCloud Drive:

• Larger Text
• Smaller Text