In my opinion it depends on the car size. I have a little Mini Cooper S with a 2L engine which I’d say is large. But in a 7-seater or large SUV that 2L would be a small engine.
Putting aside car size and just considering the engine as a stand-alone lump I’d say in the old days 5L was large but now with turbocharged small engines I’d say a 3L engine was big.
What are your thoughts?