you won’t be able to retain the original scale of billboards because the material, when used as foliage, isn’t aware of the size of each instance. if you use object bounds or object radius or any such sort of material nodes you’ll only get the size of the entire foliage ‘cluster’
there’s a node that gets you the scale of the foliage instance, but that’s only a percentage scale in relation to the original scale - not an absolute value of size