How to choose the level of theory for modelling reactions of polymers?
The question is by far not an elementary one. Short answer is: unfortunately, there is no silver bullet.

For a molecule of around 65 atoms I would go with DFT as the methodology which provides probably the best quality/cost ratio for molecules of this size.

Now, you need to choose functional and basis set, and this is a tricky business. But to start from:


Do not blindly use B3LYP. 
Try to search literature for the same types of calculations (energetics of ring opening reactions, optical properties) for similar compounds. Maybe someone already did the so-called benchmark calculations, i.e. compared on how good different functionals in combination with different basis sets are on reproducing the properties (say, specific rotation) either measured experimentally or calculated at some high level of theory (say, CCSD(T) with reasonably sized basis set).
Do your own benchmark calculations with different functionals and basis sets. First, benchmark different functionals with the same basis set (say, Dunning correlation-consistent basis sets, cc-pVTZ or better cc-pVQZ), vary the functional, compare results with experimental values, choose the best one. Once method is chosen, try to decrease and also increase (if possible) the basis set to see the effect of basis set size (with the above mentioned bases it is quite simple: if cc-pVTZ was used to benchmark different functionals, try the best functional in combination with cc-pVDZ and cc-pVQZ). Choose the smallest basis set that reproduces experimental results with desired accuracy.


P.S. You could also ask Google, there are some resources of information here and there on how to choose method and basis set, though, as I said, this question is not elementary, so only general advice can be given.