I think they are specified based on your requirements. If you want to have two showers running, the dishwasher and the washing machine running its is one size. If you just want a shower it another.

I was very interested in one but found in a retrofit the cost the unit and the installation costs made them uneconomical. However my hot water heaters are inside mechanical closets inside the house. Perhaps a basement installation would be less expensive.

In new construction I think they could be a very good idea. However I was concerned that most of the tankless units I looked at had short warranties.