The only restriction that twitter had since its inception was of 140 characters message limit.
Basically this is how this system works:
After login to Twitter, you put down your text in twitter update box with more than 140 characters and notify the application that you are ready with your message.
While some applications allows to break the message of more than 140 character into mutliple parts and update in form of multiple tweets on twitter client; though this method is less comparatively famous.
Some online web applications that provides you such facility are:
Gives you the bigger window to put your text and update the message url to Twitter client.
Twitzer is a Firefox extension which lets you post text longer than 140 characters on Twitter.
Allows the message update in parts, if more than 140 characters.