Why was the masked man the same height as Minato?
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 .