If the space is roughly a cube, then it's simply width x length x height equals cubic feet/cubic meters.
The vent fan should be rated in cubic feet per minute or cubic meters per minute.
Let's say you have space with 400 cubic meters of volume and a 200 cfm fan. It will take approximately 2 minutes to get one atmosphere exchange, provided that the fan can ventilate all nooks and crannies in the space, and that your space isolation and lockout/tagout have prevented any additional source of toxins from entering the space. Based on the rule that we try for 7 air exchanges, it will take 14 minutes to completely ventilate the space.
If the space is a cylinder, it's pi x the radius squared x the height of the cylinder to determine the volume.
Their are online calculators that let you just plug in the numbers in either feet or meters.
In New York State we use a nomograph, calculate cubic feet, factor in CFM of equipment minus friction loss from elbows etc. You can find them posted on the side of some ventilation systems or I'm sure on line. Riz