We had a MMS question from a developer regarding the maximum characters for the subject attributes and the payload when using plain text.
Q: What's the maximum number of characters for the subject field, and
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A: 64 characters is the limit to the subject size.
Q: What's the maximum number of characters for the field when using
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A: Same answer here as well. Maximum characters in the payload is based on the MMS size. I believe you can assume 1 byte per character, although I believe there is some minor overhead (approx. 200 bytes).