These values are basic things that are fairly easy to measure
such as the weight of the rocket when it is empty, the pressure
vessel's capacity and so on. Some you can measure by a number
of means such as: the capacity of the pressure vessel can be
measured by filling it with water then measuring its volume
or by weighing it.

The rocket diameter can be obtained by wrapping a piece of
paper around the rocket and measuring the length of that then
dividing that by pi.