monospace.properties — Properties of monospaced content
<xsl:attribute-set name="monospace.properties">
<xsl:attribute name="font-family">
<xsl:value-of select="$monospace.font.family"></xsl:value-of>
</xsl:attribute>
</xsl:attribute-set>
Specifies the font name for monospaced output. This property set used to set the font-size as well, but that doesn't work very well when different fonts are used (as they are in titles and paragraphs, for example).
If you want to set the font-size in a customization layer, it's probably going to be more appropriate to set font-size-adjust, if your formatter supports it.