do you mean the number of digits?

if so we want to find an integer x so that 10^(x) < 2^56 < 10^(x+1).

taking common logarithms: x < log(2^56) < x+1

x < 56(log(2)) < x+1

log(2) is approx. 0.301, so 56(0.301) = 16.856....therefore 2^56 has 17 digits.

(10^x has x+1 digits, a 1 and x 0's after)