n-bit 2's Complement Conversion
You can do this for 13 bits, or 15 bits etc, etc.. there is nothing stopping you.

The reason you jump to the next word size is because of the computers hardware. If a register has 8 bits and you are only using 6 bits, the register STILL has 8 bits. 

You can design a system with 13 bit word size, and it will still work the same.