The maximum size of a string is defined as follows:

Note that strings SHALL NOT exceed 1MB (1024*1024 characters) in size.

This definition only makes sense if we assume that 1 character is 1 byte. In unicode encodings, this is typically not a safe assumption for strings.

Therefore, I think the specification needs to clarify whether the maximum size of a string is 1024*1024 bytes (number of characters will vary depending on the encoding and the characters used) or 1024*1024 characters (maximum binary size will vary depending on the encoding and characters used)