encryption - AES How many bits should be ? (I'am learning) -

i'am research encryption aes find library , use that. , did crpyto data there things not understand

edit(15 09 2017) : sorry.. english bad im using translator words not tell want ask.. mean i'am trying learning aes encryption , find code want ask question method sorry english , have not learned yet (:

i sharing codes please tell me crypto method

1. how many bits ?
2. key random ?
3. key how many bits ?
4. how store generated key ?

i'am new yet please explenatory thank !

private static byte[] encryptstringtobytes_aes(string plaintext, byte[] key)     {         byte[] encrypted;         byte[] iv;          using (aes aesalg = aes.create())         {             aesalg.key = key;              aesalg.generateiv();             iv = aesalg.iv;              aesalg.mode = ciphermode.cbc;              var encryptor = aesalg.createencryptor(aesalg.key, aesalg.iv);               using (var msencrypt = new memorystream())             {                 using (var csencrypt = new cryptostream(msencrypt, encryptor, cryptostreammode.write))                 {                     using (var swencrypt = new streamwriter(csencrypt))                     {                         swencrypt.write(plaintext);                     }                     encrypted = msencrypt.toarray();                 }             }         }          var combinedivct = new byte[iv.length + encrypted.length];         array.copy(iv, 0, combinedivct, 0, iv.length);         array.copy(encrypted, 0, combinedivct, iv.length, encrypted.length);          return combinedivct;      } 

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private static string decryptstringfrombytes_aes(byte[] ciphertextcombined, byte[] key)     {          string plaintext = null;          using (aes aesalg = aes.create())         {             aesalg.key = key;              byte[] iv = new byte[aesalg.blocksize / 8];             byte[] ciphertext = new byte[ciphertextcombined.length - iv.length];              array.copy(ciphertextcombined, iv, iv.length);             array.copy(ciphertextcombined, iv.length, ciphertext, 0, ciphertext.length);              aesalg.iv = iv;              aesalg.mode = ciphermode.cbc;              icryptotransform decryptor = aesalg.createdecryptor(aesalg.key, aesalg.iv);              using (var msdecrypt = new memorystream(ciphertext))             {                 using (var csdecrypt = new cryptostream(msdecrypt, decryptor, cryptostreammode.read))                 {                     using (var srdecrypt = new streamreader(csdecrypt))                     {                         plaintext = srdecrypt.readtoend();                     }                 }             }          }          return plaintext;      } 

how many bits shold use ?