My answer would have been the same. When asked to work it out, I would have pointed out that I simply wouldn't; not because I don't know how, but because if I need to know the number of bits it takes to store such a specific number, there is probably a reason. That reason could be storage of some form, such as wire protocols or files, and I don't want to fuck it up because I miss a step or misremember something. So, no, I won't even bother memorizing it, let alone, calculate it by hand.