> <xsl:with-param name="length1" select="2" />
> <xsl:with-param name="sstr" select="somestring" />

In your first param, you are passing the number 2, but your second one is
looking for a child node of the element you are matching on whose name is
"somestring". Presumably you want to pass that actual string, so:

<xsl:with-param name="sstr" select="'somestring'" />

will do it.

The content of the "select" attribute is an XPath expression, and the
single quotes make it an XPath expression whose value is a string. In your
first with-param, the value of the expression can only be a number, so it
works as you expected.

HTH,

Nick.
-- 
Nick Fitzsimons
http://www.nickfitz.co.uk/