cairo:set-font-size (cr size)

cr -- a cairo:context-t instance
size -- a number coerced to a double float for the new font size, in user space units

Sets the current font matrix to a scale by a factor of size, replacing any font matrix previously set with the cairo:set-font-size or cairo:font-matrix functions. This results in a font size of size user space units. More precisely, this matrix will result in the em-square of the font being a size by size square in user space.

If text is drawn without a call to the cairo:set-font-size function, nor the cairo:font-matrix or cairo:scaled-font-t functions, the default font size is 10.0.