tl;dr: control the size of the health bar by changing its scale.
Using an UI Image is just the easiest and most lightweight way to get Unity to fill a rectangular canvas area with colored pixels. So it's the optimal thing for your use-case.
If you do not want to draw a proper health bar image yet, you can simply create a new UI image and not assign any ...
As I didn't find the ideal order for the structure that allowed Unity to recognize the existence of the bones 'head' and 'neck' (that were present), I ended up using another platform: Mixamo (check here how to upload and rig 3D characters).
I strongly recommend Mixamo as it is online, free and super simple to
Start by uploading the ...