In the question it is clearly mentioned as "The minimum size of an array that it may require to store a binary tree with n nodes" , In this case you need to take the best case possible that is Balanced Complete Tree
The minimum size is required is 2^{Hieght of Tree}-1
Hieght of Tree is Log_{2}n+1 (If you take root is in hieght 0)
The minimum size is required is 2^{log2(n+1) }-1