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A block's item modelcontact_support stores information regarding how they look whenever not placed. The "display" tag defines positions (size, position and rotation) in the following circumstances: "thirdperson_righthand":[...];
"thirdperson_lefthand":[...];
"firstperson_righthand":[...];
"firstperson_lefthand":[...];
"gui":[...];
"head":[...];
"ground":[...];
"fixed":[...]The first four cases are quite self-explanatory, "gui" defines how the block looks like in your inventory and hotbar, "ground" is how the item looks like when dropped (using Q, for example). "fixed" refers to how it looks when placed on an item frame, and finally, what you're looking for, "head" describes how the block looks like on a mob's head slot, the only normal gameplay example being obviously Carved Pumpkins. They will appear to be around the size of a humanoid model's Helmet layer, so adjusting size to be like your head would take scaling it by 0.876 in every axis (not .875 because this would cause Z-fighting artifacts)

Lastly, and unfortunately, these controls do not apply to wearable armor pieces, which all use an entity model instead.
Hope this has been useful. Good night.