OK, I figured it out - you cannot resize a folder if you only have one folder.....
I created two new small folders in addition to my original folder.
They were each big enough to hold all the data I was storing.
I then resized my original 5TB folder to a very small size 50GB (big enough to house the files I am hosting).
This worked. I then removed the original folder ( siac host folder remove <path> ).
I then removed one of the two new folders and all the stored data ended up in the remaining new 50GB folder.
I likely could have done this with just one new folder.
Happy now I can reuse the 5TB on my NAS :-)