fontSize
Font size of the code in the Playground editor and its run-output pane — any CSS length. Blank keeps Playground's own default.
Prefer absolute units ("20px"): the Playground renders in its own iframe document, so em resolves against that document's root font size rather than the surrounding slide's.
Line spacing follows automatically — CodeMirror's own line-height is a unitless ratio, so it scales with whatever font size is set here. Use lineHeight to change the ratio itself.
Throws
on assignment of a value that is not a CSS length.