If you read to the end, it has this nice quote about the coffin size from a primary source:
I eventually found a transcript of the coffin maker’s book. Here it is, in his own words: “Emlea Jane Bronty. Died Dec 19th 1848 in the 30 year of hir Age. Coffen 5ft 7” long 16” broad.”
Note that the measurement there matches the body size, not the outside of the coffin with the wood included.