Imagine how big that knife looks in soldat... then you need to figure out the size yours needs to be in proportion to it. Experimenting is the best way to learn.
Kunai knives aren't that big... it's not going to be more than 1 or 2 pixels longer than soldats knife.
Unless... you're talking about weapons for the weapons menu, in that case you need to look in interface-gfx/guns