Maybe you can do something like this?

<xsl:for-each select="Root/element[1]">
	<xsl:sort select="@length" data-type="number" order="descending"/>
		<xsl:value-of select="@length"/>
</xsl:for-each>

<xsl:for-each select="Root/element">
	<xsl:sort select="@length" data-type="number" order="descending"/>
	<xsl:if test="position() = 1">
 		<xsl:value-of select="@length"/>
	<xsl:if>
</xsl:for-each>

--
Oleg Tkachenko
Multiconn International Ltd, Israel