There's no difference in power requirements between a 15 or 21, they have the same required power for the fuel pump, fan and switch.
1.5kva is more than enough.
We have a 2.2kva (5.5hp honda) and a 2.8kva (6.5hp honda)
When we have both burners running and the van and trailer are together hooked up we run both burners from any one of the generators. When they're apart we run one burner from each.
The difference between a 21 litre burner and a 15 is the length of coil therefore the 21 is a few inches taller.