Minimums:
To get any meaningful information out of unstructured text, you need to establish some context - the more context the better the results.
Not all API calls are equal. The language identifier API call requires very little text to return a language identification while extracting entities on the other hand requires more context (i.e. news articles) to be successful.
Maximums:
The current maximum text you can process with one API call is 50k characters.