How is the rsa key size calculated?

Question

Cryptography

Let's say we have
p=3,
q=7,
{e, n} - {5, 21} (public key),
{d, n} - {17, 21} (private key)
How to calculate the key size?

Answer 1

Calculate the integer binary logarithm of d.
But usually the key length is byte-aligned. Therefore, in your case, the key length is 8 bits.