It seems based on a close reading, that there is no volume limit on how much material you can transmute.  You are limited by only two factors.  

 1. It's a single object.  The larger the object the more time you have to spend, so you can in theory spend 30 hours converting a small outhouse which is 180 cubic feet.
 2. You are able to concentrate for that long.  There might be reason to believe that you can't hold concentration for more than 24 hours. But perhaps you can.

As long as you are dealing with a single object, and you can maintain concentration, there is no limit to the volume of the object you are transmuting.