Reaction volumes vary across species and types of cells. They are expressed in terms of liters and must be entered before running both stories and simulations.

The reaction volume is used to convert rate constants that are in the units of molar into unit of molecules per reaction volume. Thus, the reaction volume is essentially a bridge between rate constants in units of concentration, and initial conditions given in numbers of molecules.  The default value of 4e-12 liters is the volume of a 20 micron diameter sphere, the approximate volume of an average mammalian cell.  A typical yeast cell has a volume of 3e-14 liters, and a typical E. coli has a volume of 1e-15 liters.

For quick reference to useful biological numbers, check out the Bionumbers Database.