  Polymers confined in corrugated channels, i.e. channels of varying amplitude,
display {multiple local maxima and minima of the diffusion coefficient upon
increasing their degree of polymerization $N$}. We propose a theoretical
effective free energy for linear polymers based on a Fick-Jacobs approach. We
validate the predictions against numerical data, obtaining quantitative
agreement for {the effective free energy, the diffusion coefficient and the
Mean First Passage Time}. Finally, we employ the effective free energy to
compute the polymer lengths $N_{min}$ at which the diffusion coefficient
presents a minimum: we find a scaling expression that we rationalize with a
blob model. Our results could be useful to design porous adsorbers, that
separate polymers of different sizes without the action of an external flow.
