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

you can declare an empty array like below

String[] arr = new String[]{}; // declare an empty array
String[] arr2 = {"A", "B"}; // declare and assign values to an array
arr = arr2; // assign valued array to empty array

you can't assign values to above empty array like below

arr[0] = "A"; // you can't do this

You can't create an array without a size. You'd need to use a list for that.

You don't have to specify the size of an array when you instantiate it.

You can still declare the array and instantiate it later. For instance:

string[] myArray;

...

myArray = new string[size];