The post length limit on Infosec.Exchange is 11,000 characters. This is possible because Infosec.Exchange runs on the glitch-soc fork of Mastodon.
The standard Mastodon post limit is much lower at 500 characters, which is still longer than Twitter at 280, but shorter than Facebook at 33,000.
With an 11,000 character limit, you could share the whole of the Project Gutenberg translation of War and Peace in a little over 300 posts, but don't.
It's fine to write longer posts, but if they're longer than a few hundred characters, put them behind a Content Warning so people don't have to scroll past your giant wall of text. Repeated or egregious use of too-long posts may collide with Rule #5.
Instance Admins have a certain amount of control to deploy their instances as they see fit, so the advice here is based on very limited testing.
If a member of Infosec.Exchange were to publish a toot in excess of the 500 character limit applied to another instance it will appear in the other instance trimmed with a Read more > notice after several hundred characters. When selected the toot appears in full, as per normal.
If posting at this length, one ought to use a Content Warning to avoid forcing users to hit a wall of text. If done, the toot will appear appropriately truncated.
This was tested with messages sent from Infosec.Exchange to another, non glitch-soc-based instance with a 500 character limit.