We have a caption limit (also for 'first comments') because we know that posts with longer captions have a harder time going up, or they fail.
When we originally didn't have any limits in place, this caused a lot of complaints from users whose posts didn't go up.
Our caption limit is 750 characters. This is long enough for 99.999% of our customers – and for most people who are reading Instagram; after all, it's not a blogging platform.
In case you have the need for particularly long captions, you can increase this to 2,200 characters (Instagram's maximum). You can do this by going to your billing/profile settings page and enabling the setting in the dashboard.
Keep in mind we do see more errors with longer captions, however the system should handle most captions.