I don't know yet either but for my first knife design I have looked at and measured hunting knives to help me determine width and length of blade and tang. I plan a skinning knife for my first followed by a drop point. I am thinking of about 3 1/2 inch blade length and a bit more for the skinner. The hunting knives I like are not long and have a max. blade thickness of about 0.1 inch. This is the thickness I plan for the drop point and may go a bit heavier for the skinner So the 1/8 thick steel I have on order is good for those, unless I don't know what I am getting into.
As for tang, I plan to try to adjust its width/taper to achieve a good center of gravity. To do this I will have to be pretty close in knowing what the other handle components will weigh.
Other than all that I don't know nothin yet.