An iris rhizome is a rhizome with leaves attached. There is no size requirement - it could be the size of a marble. A good seller will send you product that will make you happy and want to come back and buy from them again.
I do buy plants on Ebay but I keep track of my good/bad experiences. I also don't buy from sellers outside of the U.S. or sellers that have a mixed shop. If the seller is selling plants and car parts or lawn furniture or... he obviously is not in the nursery business.
Hope this helps.
