Normal Map Baking: Does Low-Poly Mesh need to have larger volume than High-Poly? This is my first character design and I'm afraid I was overly ambitious and made a mess of my vertices. I would really like to avoid starting from scratch, but I'm having troubles baking a high-poly spiky hair model onto a lower poly version (with less spikes).

Do I need to make sure that I have the same number of spikes in my low poly hair model and that the volume of the low-poly spikes encloses the volume of the high-poly spikes? Here's an image of what I'm working with for reference:
It is a bit difficult to tell what is going on from your screenshot, however, the basic answer to your question is yes - your low poly mesh needs to enclose your high poly mesh.

Your low poly mesh needs to follow all the major shapes and features of your high poly mesh. Think of it like wrapping your high poly mesh in wrapping paper. You can't have big spikes sticking out of your wrapping paper.

The purpose of baking a high poly mesh to a low poly mesh is:


To get better topology (your low poly mesh can be created with good edge flow); and
To bake all the very fine surface detail into a normal map so that its appearance is then created by normal mapping, and not actual geometry.


The trick is therefore to have your low poly mesh follow all the major shapes and features of the mesh, while ignoring the very fine detail (as this will be baked to a normal map). Sometimes that is a bit of a judgement call, but if you keep coming back to the wrapping paper analogy (what would you wrap around with wrapping paper, and what would you just paper over), it should be OK.