That depends on your measurement space. If you measure in rectangular frame then, yes, volume will change with distance. You should pick a value that is representative (e.g., 2/3 of maximum range),
If your measurement space is in spherical coordinates it will be the same no matter what the distance is.
It is also true that if you have more than one sensor, there is only one volume parameter to define. You'll have to tune it, although typically this is not where I would start tuning. The other parameters like false alarm rate and birth rate have a greater impact. Even they are only scalars and it's not ideal for a multi-sensor system.
To resolve these difficulties, we created the multiSensorTargetTracker, which allows you to define values more easily for each sensor using the sensor specification.
