Hi,
There are many ways to approach this type of problem..You can try these things...
one way is that You can use the both input and output bits as 17, by appending zeros to the input so both the input and output bits are same in number.
Another way is that your inputs are 8 bits, so you can selects the bits which are used to be the next 8 bit input out of these 17 bits output, so you can selects the msb 8 bits, or lsb 8 bits or any 8 bits from the 17 bits output ...