array of string with unknown size

Is there a specific reason why you need to use an array? If you don’t know the size before hand you might want to use List<String>

List<String> list = new List<String>();

list.Add("Hello");
list.Add("world");
list.Add("!");

Console.WriteLine(list[2]);

Will give you an output of

!

MSDN – List(T) for more information