The maximum index in any single dimension is 2,147,483,591
Remember that arrays can have multiple dimensions, so you could have a 2-D array that has up to 4,294,967,295 items, but each dimension can have a max length of 2,147,483,591.
So you can have a 2,147,483,591 X 2 array, but not a 1,000,000 X 1,000,000 array.