Cyclic polyenes CnHn generally have nonplanar geometry with no conjugation. Planar conjugated cyclic polyenes are called [n]-annulenes. To be planar they must obey some rules, namely: 1) all angles must be 120°, 2) there must be no severe steric interaction. This sets the minimum length to 18 (with exceptional length 6). Annulenes obeying n=4k+2 rule may have aromaticity (completely delocalized π-orbitals). For aromatic annulenes there is a critical n=14-30 above which there is bond length alteration. Some polyenes allow Moebius cycling, see e.g. C18H18.
Some particular cases: