Kathy,
Message objects can hold up to 4,294,967,295 (2^32-1) characters, as the 10 byte length-encoded states. An error occurs if you attempt to extend a message beyond this limit.
Regarding a field, I think the max is 100,000 chars. What you can do is looking at the conversation and if it goes beyond 100,000 chars, split them into multiply ZIBs.
Hope this helps,
Sam 8)