Unfortunately, the version of XPath and Xslt supported in .Net still 1.0 so that's what BizTalk uses.
XPath 1.0 does not have a max() function.
Since the syntax you posted suggests an Orchestration, the most practical option is a helper class that takes message as a XmlDocument parameter.
This thread has an example of hot to use an XPathDocument and an XmlDocument: http://social.msdn.microsoft.com/Forums/en-US/e6addfe7-c94f-495b-9e79-35c48844b33f/select-max-value-from-xml-file?forum=xmlandnetfx
This thread provides examples of using XDocument and LINQ: Get max attribute value from XML using LINQ