lumped port size limitation

Is there any limitation for assigning lumped port to a very small geometry in HFSS? I know that for assigning voltage excitation the size of the geometry should be larger than the wavelength. In my design there is a hemispherical shell(made of teflon) with thickness of 5 nm which is filled and surrounded by sea water. My frequency range is 1-300GHZ which gives a wavelength of 10mm-30cm. These wavelength are larger than 5 nm. Can I use lumped port to excite it? Thanks!