I am quite sure that all ammonoid eggs were less than 2mm dia., and most were less than 1mm. It is usually accepted that the eggs hatched just after the primary constriction was formed, usually found close to the protoconch, so the newly hatched ammonoids would be just slightly larger than that. This Thread on Protoconchs, discusses a lot of the ideas on egg size. How big of an aquarium would you need to house a 1 meter gelatinous ammonoid egg mass?
I would think we would need to concentrate no how SMALL an adult might be to locate one so Kevin will have to start an aquarium