When Space is set to Screen, the widget is always the same size regardless of its position in the world. There is no depth. Position in the world only dictates where to draw the widget on the 2D viewport, but then the widget is drawn as a 2D widget like it’s part of the UI.
When space is set to World, the widget is drawn in the world kinda like a decal. Then the world is rendered onto your screen. If the widget is very far it will be unreadable, if it is very close it may take up the entire screen, just like a texture on a wall. Additionally, if widget is occluded by something, then it will be properly drawn occluded. There is no occlusion in screen space.
Regarding progress bar length, you can wrap it in a Size Box to specify a desired min/max width.