According to the S&W CW-5000 Series Chiller User Manual, the CW-5000 series chillers have a reservoir capacity of 6L or 1.58503 US gallons:

Here are the volume calculations using the EXTERNAL dimensions of the tubes. The tube is actually a hollow cylinder and the water is only in the smaller center portion so i will divide by 2 in the final calculations I could have calculated it more accurately as such, but the difference should be close enough to account for the plumbing.

Here are the approximate total system coolant capacities by wattage as derived from the above information (tank cap + (tube volume/2) ).

These sound a bit high to me so i will separately calculate as a hollow cylinder and factor in the plumbing to see if there is much difference (as time allows).