Bikepacking - a viable alternative to racks & panniers

Bikepacking people, quick question.

I'm awful when it comes to working out volumes and stuff - in the real world, how much is 3 litres?

Looking at these f'rinstance, would I be able to fit, say, a t-shirt, some lightweight shorts and some light foldy shoes into 3 litres?

Could you fit that in a 2L coke bottle? If so you're fine.

Alpkit used to list the packed sizes (more or less) of their bags so you should be able to figure out how bit it is compared to your arm or thigh or something else you have to hand...