It’s not clear what you’re going for. So here’s both ways:
To make the billboard get smaller when you walk away, you have to make sure both the NpcUIand its descendants use scale.
To make the billboard stay the same size no matter the distance, you have to make sure both the NpcUIand its descendants use offset.
You must use offset values for billboards to keep them the same size.
Just set the NpcUI’s size to its offset size.
It’s pretty simple, just copy the AbsoluteSizeX and AbsoluteSizeY of the billboard and set the size to: 0, AbsoluteSizeX, 0, AbsoluteSizeY and I think that should work
quick note: you must position the camera so that the current size matches the size you want, and only then convert it to offset.