Deutsche Bahn luggage allowance
Deutsche Bahn describes what you can bring instead of publishing measurements. Their rule is shown below in their own words, and never turned into a yes or no.
What Deutsche Bahn says
Deutsche Bahn, in their own words
Luggage such as briefcases, laptops, backpacks and suitcases can be taken free of charge. In addition to your hand luggage, you are allowed to carry one additional larger piece of luggage or item free of charge, provided this load can be carried by one person alone. You do not need to reserve space for your luggage, or weigh or measure it before travelling. Because of its size, a larger item may have to be stored away from your seat, for example in the nearest luggage rack. Each passenger is responsible for looking after their own luggage. Note: Deutsche Bahn states that its luggage racks were sized around an assumed standard of 700 x 500 x 300 mm (height x width x depth), which has almost always proved suitable. That figure describes the racks, not a limit on your bag.
Before you rely on this
An empty field on this page means the operator doesn't publish that figure. It never means there is no limit.
Meeting a published limit is not a promise that staff will accept your bag at the gate. At least one operator states that paying at the gate does not guarantee a bag over the limit will be carried.