Currently, the entire size of the Message needs to be given during construction, but we may be able to avoid having the size declared using the message generics if we rebuild the message when a new word is added. So the WORD generic argument would default to 0, then a command word is added and the word is rebuilt as Message<2>, then a data word is added, and the word is rebuilt as Message<3> etc.

let message= Message::<3>::new()
        .with_command(CommandWord::new()
        .with_address(Address::Value(12))
        .with_subaddress(SubAddress::Value(5))
        .with_word_count(2)
        .build()?)
    .with_data(DataWord::new())
    .with_data(DataWord::new())
    .build()?;