Answer
The instructions that are fetched from memory are usually much larger
than $8$ bits in size, often $16$ or $32$ bits. If the $MDR$ were only $8$ bits,
then to fetch a $16-$ bit instruction we would have to go back to memory
twice. If the instruction was $32$ bits, then we would have to go back
to memory four separate times. This would greatly slow down the fetch
phase of the instruction cycle. The $MDR$ should be at least as large as the
largest instruction that we fetch.
Work Step by Step
The instructions that are fetched from memory are usually much larger
than $8$ bits in size, often $16$ or $32$ bits. If the $MDR$ were only $8$ bits,
then to fetch a $16-$ bit instruction we would have to go back to memory
twice. If the instruction was $32$ bits, then we would have to go back
to memory four separate times. This would greatly slow down the fetch
phase of the instruction cycle. The $MDR$ should be at least as large as the
largest instruction that we fetch.
