Fireflyer05 is correct. Information is available on this page.
https://www.lufthansa.com/de/en/Lufthansa-Express-Rail
After your flight lands in Frankfurt, you can collect your baggage from the Lufthansa Express Rail baggage reclaim at the airport’s mainline station, take it through customs and then directly onto the train with you.
The Lufthansa baggage regulations apply to all Lufthansa Express Rail routes. The free baggage allowance corresponds to the baggage regulations of your connecting flight.