Um, what are you trying to do? Why are you using parameters? Why not just execute SQL in code, like this:
Public Sub TestMemoField
Dim strSQL As String
strSQL = "UPDATE MyTable SET MyField='" & String(1000,"A") & "'"
CurrentDb.Execute strSQL, dbFailOnError
End Sub
I don't use parameters in saved queries except when I need to pull a value from a control on a form to be used in a saved query.
Now, my answer might not be good if your back end is not Jet or if there's something about the actual criteria and structure of your saved query that makes it important to use a saved query instead of simply using on-the-fly SQL. But you've provided virtually no information (including omitting the SQL of the querydef you're executing), so it's rather difficult to supply any kind of helpful answer.