Hello,
I have problem with wc_RsaKeyToPublicDer function when I use private key 4096 bits length.
When I use 2048 or 3072 key length everything is ok.
I generated private key by the function: 
wc_MakeRsaKey(&key_, 4096, 65537, &rng);

....
int error = wc_RsaKeyToPublicDer(&privateKey4096bits, data, data_size);

I have got error: BUFFER_E = -132,  /* output buffer too small or input too large */
I have large output buffer, so I don't have to small.
I debug asn.c  file, it seems that input is too large. But 4096 key length is acceptable by WolfSSl library:
RSA_MIN_SIZE = 512,
RSA_MAX_SIZE = 4096,

Please see my debug comments bellow.
Maybe is problem with variable leadingBit = 1, maybe should be leadingBit = 0

asn.c  file from Wolfssl libray:

/* USER RSA ifdef portions used instead of refactor in consideration for
   possible fips build */
/* Write a public RSA key to output */
static int SetRsaPublicKey(byte* output, RsaKey* key,
                           int outLen, int with_header)
{
#ifdef WOLFSSL_SMALL_STACK
    byte* n = NULL;
    byte* e = NULL;
#else
    byte n[MAX_RSA_INT_SZ];
    byte e[MAX_RSA_E_SZ];
#endif
    byte seq[MAX_SEQ_SZ];
    byte len[MAX_LENGTH_SZ + 1];  /* trailing 0 */
    int  nSz;
    int  eSz;
    int  seqSz;
    int  lenSz;
    int  idx;
    int  rawLen;
    int  leadingBit;
    int  err;

    if (output == NULL || key == NULL || outLen < MAX_SEQ_SZ)
        return BAD_FUNC_ARG;

    /* n */
#ifdef WOLFSSL_SMALL_STACK
    n = (byte*)XMALLOC(MAX_RSA_INT_SZ, NULL, DYNAMIC_TYPE_TMP_BUFFER);
    if (n == NULL)
        return MEMORY_E;
#endif

#ifdef HAVE_USER_RSA
    leadingBit = wc_Rsa_leading_bit(key->n);
    rawLen = wc_Rsa_unsigned_bin_size(key->n) + leadingBit;
#else
    leadingBit = mp_leading_bit(&key->n);                                 // I have leadingBit =1
    rawLen = mp_unsigned_bin_size(&key->n) + leadingBit;          // rawLen = 513
#endif
    n[0] = ASN_INTEGER;
    nSz  = SetLength(rawLen, n + 1) + 1;  /* int tag */                //nSz = 4

    if ( (nSz + rawLen) < MAX_RSA_INT_SZ) {                             // 4 + 513 < 517 I've got BUFFER_E  error
        if (leadingBit)
            n[nSz] = 0;
#ifdef HAVE_USER_RSA
        err = wc_Rsa_to_unsigned_bin(key->n, n + nSz, rawLen);
#else
        err = mp_to_unsigned_bin(&key->n, n + nSz + leadingBit);
#endif
        if (err == MP_OKAY)
            nSz += rawLen;
        else {
#ifdef WOLFSSL_SMALL_STACK
            XFREE(n, NULL, DYNAMIC_TYPE_TMP_BUFFER);
#endif
            return MP_TO_E;
        }
    }
    else {
#ifdef WOLFSSL_SMALL_STACK
        XFREE(n, NULL, DYNAMIC_TYPE_TMP_BUFFER);
#endif
        return BUFFER_E;
    }