ANSI X9.9 uses DES for its crypto - these keys are 56 bits long, or 112 bits for 3DES. The financial world mostly moved to 3DES a few years back so keys will generally be 112 bits. Normally they have parity bits added (1 per 7 bits of key), so are transmitted as 64 bit (single DES) or 128 bit (triple DES) values.
When presented in human-readable form, they are almost always shown as hexadecimal, so a typical 3DES key would show as 32 characters, from 0-9 and A-F. Whether you need to convert it back to binary depends on the crypto API you are using - read the documentation.
With ISO8583, usual practice is to calculate the full 64 bit MAC using 3DES in CBC mode, then put the first 4 bytes of the result into field 64 or 128 of the ISO8583 message - field 128 if any of the fields from 65-127 are present, field 64 if only fields 1-63 are in use. The final 4 bytes of the field are normally just padded with zeros.
In some legacy systems the full 64 bit MAC value may be transmitted.
I have also seen ancient systems (20+ years ago) which use ECB mode rather than CBC - basically just XOR every 8 byte block of the message together, then apply a single DES encryption to the result. This is highly insecure.