<xsl:template match="aws:Item">
...
<xsl:variable name="width" select="aws:LargeImage/aws:Width"></xsl:variable>
<xsl:variable name="height" select="aws:LargeImage/aws:Height"></xsl:variable>
...
<xsl:value-of select="$width div 2"></xsl:value-of>
<xsl:value-of select="$height div 2"></xsl:value-of>
...
</xsl:template>