For some reason, Google Translate now has a limit of 5000 characters per translation. There's even a character counter at the bottom of the input box. If you happen to paste a long text that has more than 5000 characters, you'll get an error message ("maximum characters exceeded: X characters over 5000 maximum") and a "translate more" option that lets you translate the rest of the text.
I don't understand the purpose of this restriction, considering that Google doesn't impose any limitation when translating web pages. It's worth pointing out that Google Translate's API has a similar limitation: "the maximum size of each text to be translated is 5000 characters, not including any HTML tags". Google's translation card from Google Search has a different limit: about 2800 characters.