All things being equal, short instructions are better than long ones. Minimizing size of instructions may make them harder to decode or harder to overlap. Therefore, achieving the minimum instruction size must be weighted against the time required to decode and execute instructions. Memory bandwidth is another reason for minimizing instruction length, especially with faster processors. The rate at which instructions can be executed may be limited by instruction length.
Shorter instructions mean a faster processor.
There has to be sufficient room in the instruction format to express all the operations desired. History has shown over and over the folly of not leaving a substantial number of opcodes free for future additions to the instruction set. In order to gain a finer memory resolution, one must pay the price of longer addresses and thus longer instructions. A modern compromise is to require all the bits to address individual bytes, but all memory accesses read one, two, or sometimes four words at a time. This instruction allows 2 k different operations and 2 n addressable memory cells.