I saw string-length on this website http://www.xsltfunctions.com/xsl/fn%5Fstring-length.html I think it's the xslt 2.0 syntax.
though, in xslt 1.0, i tried this:
<xsl:template match="/">
<xsl:value-of select="string-length(())" />
</xsl:template>
and i got an error. is there any function that i can use to convert () and pass it to string-length so that I can get 0?