The accumulator model allows you to set the port size, pre-charge pressure setting and nominal volume.
The pre-charge pressure is equivalent to the nitrogen pressure pre-charge and would normally be 90% of the minimum pressure.
The nominal volume is the size of the accumulator as stated in the manufacturer's data sheet. The usable volume will be much smaller than this and you can see how the volume changes with pressure during the simulation. The volume change is assumed to be isothermal e.g. it happens slowly. If the changes are fast then they would be adiabatic so the volume change available would be slightly lower. Always check with the manufacturers' or e4training.com performance calculators when specifying.