Not sure about the height issue. Apart from their centre backs Man Yoo are hardly a team of giants but they manage to defend set pieces pretty well. Evra is, what, about 5'9" or 5'10" if that, and regularly gets on the end of the ball at both ends.
I think our players have been coached to follow the man so intently that they forget to look for the ball. If you are going to man mark you probably have to do it the Stoke way, stop the bloke from going anywhere. If you chase him around, he knows where the ball is intended to go and it's easy for him to throw a feint to gain half a yard and lose the defender at the last second. And that's exactly what's been happening. It's amateurish, and whoever is coaching them this way is bloody clueless.
I'm another who thinks you need a couple of players, preferably big units, who will just watch the ball and go hell for leather for it, wherever it is directed, Laursen stylee. And I agree, Bannan should be nowhere near set pieces and Clark is a pants defender.