Yes, well, you would say that wouldn't you.
To "design" such an animal would require previous knowledge which, given you have to ask about contactor size, is obvious you have very little.
In addition there is much more to starter design than just contactor size.
Any starter/ motor combination connected to a supply authority source would need to have not only design certification to connect in the first instance but also for insurance purposes and supply authority inspection and approval to connect.
Not to mention required electrical licencing.
Especially given 100hp motor.
You may get away with an old washing machine motor plugged into a power outlet but not what you are suggesting.