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.
Edit: Let me add some context to my observation to help you understand why I find the information about the length restrictions useful.
Recently I've tried to publish a very long post on Lemmy. As a new user to the platform I initially assumed that there is a lot of traffic on the server and because of that my post takes longer to publish. After some time without any success publishing the post I assumed that there is a server issue and I decided to try again later. A few hours later my post still didn't want to publish so I assumed that there might be a length restriction that originates from the basic principles of the fediverse (i.e., instances communicating with one another sending copies of their posts and comments to other instances).
To test my hypothesis about the length restrictions I tried posting only the first paragraph of my post. A few seconds later the post was already online. Then I've started adding one paragraph at a time and hitting publish until I've encountered the same issue (i.e., submission pending indefinitely). When I've analysed the edited post I've realised that it was almost 10000 characters long.
Had I known from the beginning about the length restriction, then I would have saved a few hours trying to publish the post by splitting it in parts and posting the parts as comments.