Rack pulls can be used for both size and strength gains (the distinction between the two is a bit vague anyway). It will probably depend more on the rep range you're using than the height of the bar. I rack pull from mid-shin every few weeks to train my deadlift, but I do 1RM's as I'm only concerned with strength gains.
It's not uncommon to lift a lower weight from this position than you do off the floor, you lose some of the leg drive that you would get when pulling off the floor, without getting a significant mechanical advantage in the way you do for higher racks pulls.
I'll admit that I know far more about training for strength than I do for size, but I would recommend that knee height, or just below, be your upper limit on rack pull height. From a strength training point of view I don't let anyone I coach do rack pulls from a height that allows them to do more than about 10% more than they pull from the floor. If you do then you're wasting your time preparing your body for weights you won't handle for a long time.
And you should definitely drive your hips through, regardless of why your doing these.