For your experiment, you would need to see decide what your hypothesis is before you design the experiment. If you want to see if the volume of gas changes depending on the container it's in, then you need to set it up depending on what you want to do with the volume.
The manipulated variable would be whatever you change, so if you switch container sizes, that would be the manipulated variable and the responding variable would be the volume of the gas.
I hope this helps!