I will be building a 100' long home with water usage at the two ends of the
house.

At one end will be a mostly over the garage apartment for my daughter with
kitchen, bath, laundry. Also there will be a bath with on the first floor
with our kitchen and also laundry and utility tubs in the laundry and
garage.

In the middle will be a 2nd floor guest bath.

On the other end will be a first floor master bath with shower and 72"
jetted tub (Venus II).

A priority is not to have to wait "forever" for hot water to reach a
spigot, especially at the kitchen & lavatory sinks and utility tubs.

I am planning to have a hot water heater in the garage and another under
the master bath (crawl space here will be about 6 feet high due to land
slope).

How large should I make each water heater. Should I have a separate
additional heater for that huge tub and only turn it on when needed?

Many thanks for any knowledgeable help!