SSL_read() and SSL_write() are limited to int. The new SSL_write_ex() and SSL_read_ex() APIs support size_t just like read(2) and recv(2). Also see bpo-42853.

int SSL_write_ex(SSL *s, const void *buf, size_t num, size_t *written);
int SSL_read_ex(SSL *ssl, void *buf, size_t num, size_t *readbytes);

Both functions return 1 for success or 0 for failure.