The only maximum I know of is the max 100' for the cable (of course you should have the Tstat in the same building!
). Since every installation is a little different I would recommend starting in the location you want to try and regulate, temporarily mount the Tstat on the wall at the proper height (make sure it is not adjacent to any of the ducts from the stove) and then see how it works.
Since the Coal-Trol is designed for the slower reaction times of coal, I believe it is possible to do what you are wanting to do. With a little trial and error I bet you will find a good spot that gives you a warm upstairs and an even warmer downstairs.
Have fun with the science project!