Use this link to help select the appropriate size  whole home standby generator for your customer. Keep in mind that you CAN use a transfer switch that is rated higher than the main breaker panel. A 200amp transfer switch is acceptable on a 100amp service, but, a 100amp switch on a 200amp panel is not allowed. 

https://www.generac.com/for-homeowners/home-backup-power/build-your-generator