How can humans hear low frequency sounds using the cochlea which is tiny compared to wavelength?
You are thinking of antennas for EM signals such as the dipole antenna. In this type of antenna, an external field (EM waves) excite the antenna (creating an electric current) which you then measure. There is thus something like a transduction step, which is more efficient if you can use the spatial scale of the wavelength.

In the case of hearing sound waves, the ear does not transduce the mechanical signal into another modality, but is directly modulated by the hair cells.