The maximum length of each translated string through Translator API

Hi,

I'm calling Translator Web Service from local program. Thinking that there should be no upper bound on HTTP POST body, I fed in a string of length around 12000. Then exception message says: The remote server returned an error: (400) Bad Request.

Then I did some tests and found the boundary is around 10X00. If more than this level, server would return the error above.

So is there official explanation of such error and is this really a "defined" value?

Thanks so much for your help!

Best Regards,
Lynn