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iMessage will reportedly dodge EU regulations, won’t have to open up
(arstechnica.com)
This is a most excellent place for technology news and articles.
There is no such specification. It is solely up to the provider. For example, T-Mobile and ATT both state 100MB on their "Advanced Messaging" FAQ. I'm sure Verizon is the same though I couldn't find the exact wording.
Photos and Videos through RCS use your data.
The spec does state there is an 8,000 character limit and a maximum of 100 participants in a group conversation.