As WyliesDiesels said, make and model are important to know for sure.
The one I installed for my in-laws is a 20kW Kohler unit with an automatic transfer switch. They typically come with a 200A ATS but without any breakers... and that didn't work for what we wanted to do. So instead I changed the order from a 200A ATS to a 100A Service Entrance Rated ATS that comes with a 100A main breaker. That way power comes from the meter, into the ATS, then feeds the house as a sub-panel.
The E/G is only good for about 85A so we're not even close to the limits of the rest of the system... but again that's based on this particular project. Yours may be different.
My question would also be: Do you intend on feeding the whole house and installing the ATS between the meter and the main, or installing it after the house panel to feed a sub-panel with specific loads designated in your house and moving those circuits over to a new panel?
What would you ultimately like to accomplish?
Best regards,
Mark