I think this way would be better:

#if (NGX_HAVE_UNIX_DOMAIN)
#define NGX_SOCKADDRLEN sizeof(struct sockaddr_un)
#else
#define NGX_SOCKADDRLEN sizeof(struct sockaddr_storage)
#endif

sizeof(sturct sockaddr_storage) is 128 in my Linux 64bit, 512 is too big.

anyway, this is not critical.