Soakaway size is calculated, not standardised. The storage volume required depends on the area of roof and paving draining to it, the design rainfall event, and the measured infiltration rate of the soil. An engineer performs this calculation using percolation test results for the specific location.
The principle is simple: the soakaway must store the run-off from a design storm faster than it arrives, and empty again quickly enough to be ready for the next event. That means comparing inflow from the contributing impermeable area with outflow through the sides and base of the soakaway at the measured infiltration rate.
Poorly draining ground requires a much larger structure for the same roof area, and at some point infiltration becomes impractical. Guesswork is the usual cause of soakaways that overflow, so the calculation should be documented as part of the drainage design.