Is this an appropriate length for a quote?

If I were you I would not alter the quote at all. For starters it does not seem overly long and if it is relevant to your argument and backs up your line of thinking then it is fine. The only reason tutors like myself tell people quotes are too long is generally (a) becuase vast swaths of it are not relevant (2) it is far too long for the length of the essay (3) it replaces rather than backs up your argument. The quote, as a rule, should back up what you are saying rather than effectively making the argument for you. Hope that helps :-)