For the prototype, the batches used have a mass of 10 kg. This number does not have any scientific importance except that is an easy number for calculations. Lemongrass has moisture content of 50.0%, so one batch holds 5 kg of water and 5 kg of bone dry material. We can neglect the amount of lemongrass oil in these calculations, because this is very small amount compared to the 10 kg.
The distiller should be a cylinder, so we want to know its measurements. The density (bulk) of the grass ρ=160 kg/m3. Then the steam volume required mass of the grass divided by the density Vsteam= 0.0625 m3 = 62.5 l. We estimate a 20% marge in the volume, so our design volume of the distiller Vdesign= 0.075 m3. We want a height to diameter ratio of 3, because these are common proportions for fabrication. This gives us a height H=1.2 m and diameter D=0.4 m for the distiller.