You don't think he gets to decide the running length? Or at the very least have some say in it?
On the one hand, I'm sure he gets some input, but on the other hand there's the practical limitations that are imposed.
What will fit in the schedule? And, how much time and resources did the BBC give him to produce the episode?
Those factors all limit the potential length. And, I'm sure the 3D nature of the production imposes more limitations on whatever resources they did get.
Just saying he can't pick a length willy nilly. I don't think it'll be too far off from the original 60 minutes. Maybe 70.