some things to consider:

Vessel packing. If you have a catalyst or some material to be extracted that is within the vessel, that will change the amount of volume that is available for you to flow through your vessel. The length and diameter of the vessel depends on the desired fluid pressure and velocity and amount of free space available in the vessel after packing with catalyst or other material. Say I have a feed condition of 500gms H2/min at 350K and 2 bar. for simplicity, we will assume no packing in this vessel. You will use the appropriate gas law to determine the volumetric flow rate of this mass through your vessel, instead of using a fixed v and n you will just use the gms per minute to convert to volume per minute. the fluid velocity is calculated as the volumetric flow rate divided by cross sectional area (thus diameter) of the vessel. the dwell time within the vessel will be determined by the length of the vessel divided by the velocity of the fluid. now that you've designed for a certain velocity (diameter) and dwell time (length), you can get your vessel volume.