It is actually a quite simple script. It only counts how many A, C, G, Us there are and uses the formula:

"Exact" calculation

volume (cubic Angstroms) = #A*315.45 + #C*291.285 + #G*323.028 + #U*291.285

mass (Daltons) = #A*329.2 + #C*305.2 + #G*345.2 + #U*306.2

For an RNA that size the average might just as good:

7.4kB * 304 A^3 ~= 2.25 million cubic Angstroms

7.4kB * 321.45 A^3 ~= 2.38 MD (MegaDaltons)

PSV is something like volume/mass*0.6022 ~= 0.57