Entropy as an intrinsic property of matter
You can use the tools of statistical mechanics to compute the entropy of molecules in thermal equilibrium. 

For molecules in an ideal gas the molar entropy is
$$S=R\ln z +RT\left(\frac{\partial z}{\partial T} \right) _V - R \ln N_A + R$$
Here $z$ is the molecular partition function
$$z=\sum_i e^{\epsilon_i/kT}$$
where the summation extends over all states available to a molecule and $\epsilon _i$ is the energy of state $i$ (states degenerate in energy are counted separately). 

If you can identify all the possible conformations ("geometries") available to the molecule and their respective possible energy states, you can use the formula to compute the entropy.

Qualitatively speaking, if you increase the number of conformations you increase the entropy, since the summation in the partition function now extends over more states. 

You might invoke Boltzmann's equation 
$$p_i=\frac{e^{-\epsilon_i/kT}}{z}$$
to compute the probability of finding a particular molecule in state i. 

Reference

McQuarrie and Simon, Statistical Thermodynamics, University Science Books (1999).