You can't do this in a Listbox, it'll be limited to the first 256 characters.
In a textbox you'll need to "zoom" it to show the content. Assuming you're working with a Single View Form (where one record at a time is visible) which is really the only way a record with a memo field should be displayed, this will do it:
Code:
Private Sub YourControl_GotFocus()
DoCmd.RunCommand acCmdZoomBox
End Sub