Understanding word and instruction on simple microcontroller
Word width is fixed. It is 256 words of 12 bits each. You cannot convert this to bytes as it is not on byte-sized boundaries.
Instruction cycles on most all MPU's and CPU's is 1/4 the clock rate. If clock is 4 MHZ then it can spew out instructions at a 1 MHZ rate. Note that more complex MPU's have multiple clock sources and also sleep-modes where another slower clock is used. In some sleep modes the MPU core may be shut down.
Note that some ops require extended clock cycles to complete, so as Dave Tweed mentions in his answer read the datasheet in detail.