Not published: minimum installation room volume (cubic feet)
Neither DOE CCD nor ENERGY STAR publishes a required air volume for a heat pump water heater's install space — only its physical footprint and its ambient operating temperature range, which measure different things (a room can clear both and still be smaller than a manufacturer's own install-manual minimum, which this registry does not carry either).
This registry does publish ambient operating range and physical footprint for heat-pump models — 320 of 345 in this catalog file an ambient range today.
Not published: models with a filed footprint in this scope
None of the models on file in this scope carries a filed height or footprint in the registries this site reads.
Check the water heater selector for today's full catalog.
Enter your own available height, width, depth and (for a heat pump model) ambient temperature on the water heater selector to check the full catalog against your own space, not just the models shown above.