In FiledMetaData it is stated:
If the dataType field is a String, LocalizedText or ByteString then this field specifies the maximum supported length....

I assume that this number specifies the number of characters / bytes in encoding for the text part (not including the 4 byte of length).
=> A string with maxStringLength of 20 shall be encoded as
4 byte: string length (e.g. 5)
n byte: string content (e.g. 5 byte)
20 - n byte: padding (e.g. 15)

Or shall the maxStringLength define the "field size", which would change the number of padding bytes to 11 in example above.
For localized texts additionally 2 byte for namespace index would restrict "real" string length.

What is not clearly defined as well:

• shall this padding be done for RawData encoding only or in general?
  I would assume yes; otherwise data type LocalizedText would not make any sense.
• if a string does not fit into the specified length, how shall be reacted?
  • truncate the string? (unlikely)
  • invalid data set message? (IsValid bit is set to 0)
  • ...