master.xml

<data>
    <thumbs>
        <item>120</item>
        <item>240</item>
        <item>360</item>
    </thumbs>
</data>

master.xsl

<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:exsl="http://exslt.org/common"
    xmlns:math="http://exslt.org/math"
    extension-element-prefixes="exsl math">

<xsl:output method="xml" indent="yes" />

<xsl:variable name="widths">

        <item>120</item>
        <item>240</item>
        <item>360</item>

</xsl:variable>
      
<xsl:template match="/">
    <xsl:call-template name="find-random-node"/>
</xsl:template>

<xsl:template name="find-random-node">
  <xsl:variable name="nodes" select="exsl:node-set($widths)"/>
  <xsl:variable name="node-count" select="count($nodes/item)"/>
    <xsl:variable name="random-node" select="ceiling(math:random() * $node-count)"/>

    <count><xsl:value-of select="$node-count"/></count>
    <random><xsl:value-of select="$random-node"/></random>
</xsl:template>

</xsl:stylesheet>