Currently there is no limit to the number of characters that a post can contain. However, this value is set strictly in the database, and since a LONGTEXT field in the database can only hold a little over 5 million characters, this is where the limit stands.
The content box itself will display around 500-1000 characters before prompting the "Read More" link, which will expand the full content of the post. The character limit does not include pasted or inserted links and videos, as reference locations to these are stored elsewhere in the posts table.
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