PML width for Longer wavelength

I generally use the wavelength(in case of shorter wavelength)as the starting PML width. This does not create a large geometry to solve, saving some memory. But, my recent models have longer wavelengths. These are several magnitudes higher than the resonators actual size which I am modelling. Creating a PML as large as the wavelength does not make any sense. What could be a better way to find a starting PML width?

Thanks.