man RSA_size () - get RSA modulus size
NAME
RSA_size - get RSA modulus size
SYNOPSIS
#include <openssl/rsa.h>
int RSA_size(const RSA *rsa);
DESCRIPTION
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.
RETURN VALUE
The size in bytes.
SEE ALSO
HISTORY
RSA_size() is available in all versions of SSLeay and OpenSSL.