Don't expect a guarantee from most peony growers. The only guarantee most will give you is that the plant they sent you is the plant you received. There are too many variables in tree peonies for most nurseries to give any sort of guarantee other than it was alive when it left their hands.

I ordered 6 Chinese tree peonies from them 2 years ago, and was quite pleased. The plant size depends on what you are willing to pay for -- I purchased one 5-year old plant ($$$) which had several stems, quite a large root system, and is currently close to 4 feet tall. The rest were all 2-3 year plants, and were much smaller.
I don't know about the guarantee, since I didn't have any issues with my plants. YMMV, of course.
I would buy from them again if I had any space left. I'll have to tear up more grass sometime soon.

Check out watchdog for reviews on this company also solaris Farms has some of the best tree peonies I have recieved and would highly recommend Huge Plants!