How do I determine the size of a dynamic array?

[RusselLuke]

I'm trying to find the sum of all the digits that make up the number 2^1000, which you may have guessed is from Project Eula. I think it's good so far except I'm just learning about arrays and dynamic arrays, so how will I know what the size of the index will be so I can specify when the Do loop should stop so I don't get an indexoutofrange exception?

Dim BigNumber As Double = 2 ^ 1000
        Dim DigitArray() As Char = BigNumber.ToString.ToCharArray
        Dim Index As Integer = 0
        Dim SumOfDigits As Integer = 0
 
        Do Until index
            SumOfDigits += Integer.Parse(DigitArray(Index))
            Index += 1
        Loop
 
        TextBox1.Text = SumOfDigits.ToString