Aha! I cracked the code.
Open the charms and search for "text size", then select "make text and other items smaller or larger" as suggested above. You'll notice all the options are greyed out. Now check the box that says "let me choose one scaling level for all my displays" and click Apply, then change the scaling level and click Apply again. You'll have to log out for the change to take effect.
Ahh. Much better!