Does the Initialization Vector in DES have to be 8 bytes long? I was experimenting around with DES and decided to implement an IV (Initialization Vector) for it. I noticed that in DES, which is 56 bits and 8 parity bits, the IV would be 8 bytes in length. Does that mean that if I were to program DES but using a 128-bit key, the IV would be 16 bytes in length instead of 8 bytes in length? (Cryptography Rookie here)
The IV is part of the block cipher mode of operation, not of the block cipher itself. The block cipher simply doesn't have any input for an IV, it just takes an input block and produces an output block using a key. So you cannot implement an IV for DES itself.

The block size is however usually of influence to the mode of operation. Even then, the IV may still be different from the block size for some modes. To make it a bit more complex, some modes of operation such as CTR mode and GCM mode (for 128 bit block ciphers such as AES) may have different options to implement an IV or nonce.

The DES block size is always 64 bits, regardless of the key size, just like 128 bits is always the block size of AES. The key size is not (necessarily) of any influence to the block size, although it may be advantageous for the key size to be a multiple of the block size in case keys need to be encrypted by other keys (wrapping a key by another "wrapping" key).

For CBC mode, which is often explained as one of the first modes to starting cryptographers, the IV needs to be the full block size. Furthermore, for CBC the IV needs to be indistinguishable from random to any adversary. That means that the IV is always set to 8 bytes or 64 bits.



Note that DES itself only supports 64 bit keys, of which 8 bits are parity bits, so an effective key size of 56 bits. Triple DES or DES-EDE may use two keys. It then uses key A for the first encryption, key B for DES in decryption mode and keyA again for encryption, so an ABA-key.

However, as these block encrypts / decrypts are simply stacked on top of each other, the block size stays the same: 64 bits. Therefore, for CBC, the IV does need to be 64 bits / 8 bytes as well.