Why was the masked man the same height as Minato? Tobi/Madara aka the masked man was revealed to be 


   Obito Uchiha.


This was revealed in the Fourth Great Ninja War but I cannot how the masked man could control the nine tails? At that time, Kakashi, Guy and his other peers were 15 years old more or less, he should also be around that age. Why do we see that the masked man is around Minato's height?
Note: Minato was at this time a full grown adult in his 20s.

Even for a very skilled shinobi, I believe it's impossible to learn so much in just 2 years. Obito was about 13 and a mediocre ninja when he was presumed dead in Konoha and encountered Madara. Now, I know many of you might say that he was taught by Madara but at that time Madara was himself and his 80s, very weak, and his life was attached to the Gedo statue. Now in the battle between the masked man and Minato we clearly see that when Minato hits the rasengan to the masked man a whitish DNA sort of thing drips from his arm. That could have been Obito's senju DNA as his original right body was crushed but now I couldn't agree to this because of the first paragraph. Perhaps I am missing something else.
Obito had a part of zetsu attached to him , that explains the immense chakra to summon nine tails and also the height . 
When the cave collapsed and the boulder fell on obito ,half of his body was under it and the other half was not after which he decided to give kakashi his sharingan . From the manga-599 onwards upto 605 , we see the entire incident happen , from the cave collapsing to obito meeting madara for the first time , he is also not able to support his own weight and hence part of zetsu and attaches himself to him and they train together since the attachment is not entirely stable . 
Another reason for obito being able to summon the nine tails fox could be because he activated the mangekyou sharingan at such an early age .