I measured my kiln and it is 17.5" octagon 18.5" deep. There are much larger kilns though. Automated ceramic kilns are getting pretty popular in the ceramic industry and non automated kilns are going pretty cheap. Used ceramic stuff in general is pretty cheap.
The nice thing about a kiln is it warms fairly quickly and you can do some higher temp coatings. I can fit exhaust from my MGB in this...It has a Chevy V6. That is about as large as I could put in here. I am going to make a stand to support things which sits in the bottom.