Actually it would be the materials. The ratios would not change between concrete, brick, stone, or cob, just the thickness perhaps with softer materials. Modern, prefab, concrete, woodfired ovens are subject to the exact same ratios as is the builder of the cob-clay woodfired oven. My neighbor has a woodfired oven in his back yard that is a copy of a Renaissance version he saw in Italy..., the ratios are the same for the clay oven that Jas Townsend builds in his videos. The same would be true for the wood grill that had a chimney, or copying a hearth & chimney for cooking from an 18th century home.
LD