Not to speak for Nyeti or Craig, but different schools of thought...
A longer blade creates a greater wound channel and will reach vital organs, a spear point is especially good at this. However, you may not have the option to legally carry a double edged blade where you live. Look at knives that people hunt game with and you will find very few "tactical" designs. When your life is literally on the line, traditional designs that have worked for centuries are used most.
A 3.5"-4" blade will work well (on humans) and is easily concealable with the correct sheath.
No matter what knife you choose, training with a good instructor will be the most important factor. Do lots of research and look at many knives before you commit to any one martial arts system or blade design. Find something that works best for YOU.