Mine isnt a clarke, but this one is basically the same -
If you can find a picture of the end (or download the clarke manual) you will see the filling / level hole - it would be on the right hand end of the beam in the photo above.
From that, you could get an idea of the volume, i guess. Again, the beam is only about 1/2 full of oil, if that. There isnt much more oil stored in the rest of the system (my unit was purchased broken, so i was familiar with it at the time - but that was a few years back now!)
It talks about "dipstick" - i thought it was just a filler / level plug, but it seems not. If there's a dipstick, it means more of the beam is full of oil than i thought.
The manual specifies 3.5ltrs. I guess if you really wanted to copy that unit, i could measure up the beam, and the dipstick length (which will give you the air volume) and you can see if all the numbers match up.
...the reason i seem reluctant is that i know the machine is behind a whole load of stuff at the moment, and it will require stuff to be moved to even touch it!
But you can go lower. I am not too sure how duty cycle effects it as the tank is mostly for removing air. Copying the capacity of a tiny electric one is a bad idea.
Could you get a smaller pump as 8gpm might be a bit overkill