1. I can see that you have non formatted long texts. Can you separate long texts with many sentences by wrapping sentences with <span> tag . Ex. <span>Sentence 1</span> <span>Sentence 2</span> etc
2. There is no document size limit. There is 100.000 character daily limit. So every day 100KB of your site will be translated and cached.
3.API key is an optional feature and GTranslate Pro works without API. The google API key is a commercial and if you want you can get google API key from https://code.google.com/apis/console/?api=translate
. The Bing API key has some free limits.