Every service has different criteria for how long a status update can be, and on X.com you can have a longer status if you’re a premium subscriber. However, XPoster only has one setting for this, and it’s shared by all status updates. The default length is 280 characters (or 140 characters if you’re operating in Chinese, Japanese, or Korean.)
If you’re posting to X.com and are not a premium subscriber, this is the highest you can go.
The maximum allowed on Mastodon is usually 500 characters, but could be different in some rare circumstances. There are no mechanisms to adjust this in Mastodon; but it is open source, and the limits could be adjusted by the server admin.
The maximum allowed on Bluesky is 300 characters.
If you set your character length over 280 characters, XPoster will display a warning about character length, but will not stop you from saving the setting.