Shields?
 

How exactly do shields work? I've seen huge shields, and smaller shields; what are the rules for how to create one? Do they have to be a certain dimension? Do they follow a template (e.g. heads/bodies)?

It's percentage-based, I think, of the dimensions of the shield. Take shield1.png, and find the percentage that each direction takes within the dimensions. Use those percentages. Or just resize it and use it as a base.

template

I don't think they follow a template, as Chris said, there are quite tons of shields all different, maybe it's a template percentage but looking at wings and a single shield, it's just weird to figure :/

I tested this and each direction of the shield is indeed a fixed percentage of the image dimensions. You'll be better of using Dusty's advice and resizing bigshield.png to a desired size and using it as a template. I created one and attached it below for convenience. The directions are up, left, down, right respectively
Graal doesn't seem to care how large you make the shield - just as long as the directions are set appropriately