I've grown my hair long in the past, and I know from experience that I get about the same number of tangles whether it is long or short.  If it is short, it is far curlier and therefore tangles more easily, but if it is long there is more of it.  I have found that the longer it gets the more likely I am to put it up (otherwise it gets in the way) and if it is up in usually tangles less.