You would want to calculate the threshold by choosing your target epsilon and delta and an 'm' equal to the largest conceivable size of the stream. Fortunately,

You would want to calculate the threshold by choosing your target epsilon and delta and an 'm' equal to the largest conceivable size of the stream. Fortunately, the threshold increases with log(m), so it's inexpensive to anticipate several orders of magnitude more data than necessary. If you wanted, you could work backwards to calculate the actual 'epsilon' and 'delta' values for the actual 'm' of the stream after the fact.