Firstly Doombunny - this is not Nagash at the height of his power, far from it. At the height of his power Nagash would have crumpled Sigmar with barely an effort, he wasn't just a demi-god like Sigmar, he was pretty much the full thing. It took Nagash being killed once, loosing a lot of his power coming back from Death, lossing some power because his soul is now split in three (his body, claw & crown), and even then Sigmar only just beat him and suffered a grevious wound in return. No one has ever faced Nagash in his full power, the first time he was killed he was basically comatose.
So back on subject, this is Nagash in his current weakened state, in which the 4th edition showed him being on par with Greater Daemons.
In regards to his I, I am working off the fact he was an extremely skilled warrior in life and death, the skills of which he hasn't forgotten, and also looking at the stats created for him in the 4th edition showed him being that fast. No doubt his skills honed over many many years (don't forget Nagash fought hand to hand with Sigmar and only just lost), and also as MS said, no doubt his magically enhanced body helps.
About him being a large target, yes he should be, an oversight on my part that will be amended, and no he will not benefit from look out sir! due to his size. In fact after thinking over it I have created the Warped Monstrosity rule.
I have amended his sword to show he heals one wound per wound inflicted.
How does it look now?
Oh, and Doombunny, Arion is quite right. GW took the official stance a few years back that the Fantasy and 40k universes are not connected in anyway, so any information prior to that is now incorrect.