Actually, I've just documented this here as a new entry in the "additional information" list for visual markings, because we discovered Excel has a much shorter max string length for headers/footers (253 characters) - so the string can truncate if the number of characters that you specify for the label exceeds this number.  

 

To answer your original question:

  • Max number of characters you can specify for headers/footers: 1024
  • Max number of characters you can specify for watermarks: 255