...
- Apache 2.0.53 and later: http://httpd.apache.org/docs/2.0/mod/core.html#limitrequestfieldsize
- IIS: http://support.microsoft.com/default.aspx?scid=kb;en-us;310156&sd=tech
However, if you still experience errors, there isn't any known workaround that will guarantee success. Consider upgrading to Shibboleth 2.0, which has a more robust mechanism for export.