I need to find a place to leave my luggage in lockers for 2 days in Frankfurt, either the airport (Terminal 1) or at the train station. I have to go by train to Stuttgart for 2 days without luggage. The luggage is 5 bags, 28 inches.
There are lockers at Frankfurt Main Station (Frankfurt Hbf). You can leave your bags there. A small locker costs 3 or 4 Eur per day, a big locker costs 5 or 6 Eur per day. Note, that day ends at midnight, it doesn't matter when you put your bags in the locker - at 3:00 or at 23:00.
You can also store your bag at the airport for up to three months. Here is the information about locations, prices and working hours.