man DH_size () - get Diffie-Hellman prime size
NAME
DH_size - get Diffie-Hellman prime size
SYNOPSIS
#include <openssl/dh.h>
int DH_size(DH *dh);
DESCRIPTION
This function returns the Diffie-Hellman size in bytes. It can be used to determine how much memory must be allocated for the shared secret computed by DH_compute_key().
dh->p must not be NULL.
RETURN VALUE
The size in bytes.
SEE ALSO
dh(3), DH_generate_key(3)
HISTORY
DH_size() is available in all versions of SSLeay and OpenSSL.