It seems that posts on Lemmy cannot exceed 10000 characters. If you type in more than 10000 characters and try to submit the post, you will see that the post submission is being processed but you won’t come any further than that. The post submission will remain in status pending indefinitely.
And I believe the limit on comments is 5000 characters, based on my having to split a paste of text from a reddit submission into two comments and being notified of that.