To pass string parameters to xsltproc, use a command like the following (possibly with --xinclude).
xsltproc -o animals.tex --stringparam latex.font.size "20pt"

You can use as many stringparam as you like on the command-line (or in your scripts). The quotation marks are not strictly needed in this example, but if the value of the parameter has spaces, slashes, etc., then you need to quote-protect the string from the command-line processor, and either single or double quotes will work (and protect the other kind).