What part of an RSA key is referred to by the number of bits? When we speak about 4096- or 2048-bit RSA keys, what part of the key is this number of bits? The public key comprises both the modulus and the public exponent, and the strength of the key can also be seen as depending on the length of the prime factors. Which of these is the "4096-bit" part?
See https://en.wikipedia.org/wiki/RSA_%28cryptosystem%29

Under Key Generation:

Compute n = pq.
    n is used as the modulus for both the public and private keys. Its length, usually expressed in bits, is the key length.