RSA_size

 #include <openssl/rsa.h>
 int RSA_size(const RSA *rsa);

This function returns the RSA modulus size in bytes. It can be used to determine how much memory must be allocated for an RSA encrypted value.

rsa->n must not be NULL.

The size in bytes.

rsa(3)

RSA_size() is available in all versions of SSLeay and OpenSSL.