> Как определить размер принятого пакета функцией recvfrom() ?

пробовать типа recv_len = recvfrom(....)

цитата из man:
ssize_t recv(int sockfd, void *buf, size_t len, int flags);

ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
                 struct sockaddr *src_addr, socklen_t *addrlen);

ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);

All three routines return the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from.