Barbara,
The (monomer?) chain length of PDMS is variable. It is possible to
purchase PDMS of many different monomer lengths via the viscosity of
the solution. The relationship between viscosity and molecular weight
is available in the literature:

    Molecular weight dependence of low shear rate viscosity for
poly(dimethylsiloxanes). UTC =3D Silicon Compounds: Register and
Review,    5th ed., United Chemical Technologies, Pennsylvania (1993);
Lee et al =3D    C.L. Lee, K.E. Polmanteer, E.G. King, =93Flow Behavior
of    Narrow-Distribution Polydimethylsiloxane,=94 J. Polym. Sci., A-2,
8,    p1909-1916 (1970); Barry =3D A.J. Barry, =93Viscometric
Investigation of    Dimethylsiloxane Polymers,=94 J. Appl. Phys., 17,
p1020-1024 (1946);    Mills =3D N.J. Mills, =93The Rheological
Properties and Molecular Weight    Distribution of
Polydimethylsiloxane,=94 Eur. Polym. J., 5, p675-695    (1969);
Rahalkar et al =3D R.R. Rahalkar, J. Lamb, G. Harrison, A.J.    Barlow,
=93Viscoelastic Studies of Reptational Motion of Linear
Polydimethylsiloxanes,=94 Faraday Symp. Chem. Soc., 18, p103-114
(1983);    El Kissi =3D N. El Kissi, J.M. Piau, P. Attan=E9, G. Turrel,
=93Shear    Rheometry of Polydimethylsiloxanes. Master Curves and
Testing of    Gleissle and Yamamoto Relations,=94 Rheol. Acta, 32,
p293-310 (1993); Dow    Corning =3D Dow Corning fluids product
information.