Why are all data type sizes always a power of 2?
Let's take two examples:
short int 16
char 8
Why are they not the like following?
short int 12
That's an implementation detail, and it isn't always the case. Some exotic architectures have non-power-of-two data types. For example, 36-bit words were common at one stage.
The reason powers of two are almost universal these days is that it typically simplifies internal hardware implementations. As a hypothetical example (I don't do hardware, so I have to confess that this is mostly guesswork), the portion of an opcode that indicates how large one of its arguments is might be stored as the power-of-two index of the number of bytes in the argument, thus two bits is sufficient to express which of 8, 16, 32 or 64 bits the argument is, and the circuitry required to convert that into the appropriate latching signals would be quite simple.