It's hard to give advice when I cannot see your blog and theme. You have not provided your wordpress.com blog url and you have linked something other than your wordpress.com blog to your username so this is like stabbing in the dark ... sigh...
There is no length to post limit. You can change the number of lines in the editor box here -> Options -> Writing
Size of the post box: ____lines
(increase the number of lines and click "Update Options" to save)
If this does not rectify the problem please post again and include the url to the blog in question.