In the classical world, a radio antenna designed for operation at a certain wavelength must be close to the same size as the wavelength – typically within about one order of magnitude. Otherwise, the antenna will not work efficiently. Intuitively, one might imagine that low-energy photons are "too large" to be absorbed by a small antenna and will simply pass through. (Admittedly, this argument is rather misleading and shouldn't be taken too seriously.)
This limitation doesn't apply to atoms. A typical atomic orbital might have a characteristic length scale of a few ångströms, yet atoms often absorb and radiate photons with wavelengths as large as a few hundred nanometers. For their sizes, atomic "antennas" can be surprisingly efficient. For example, with careful experimental design, a single atom can block as much as 3% of an incident laser beam.
Comparing these two cases motivates me to wonder about what happens at intermediate scales. In a single atom, energy levels with large transition probabilities are normally separated by perhaps an eV (plus or minus a couple of orders of magnitude). Therefore, atomic spectra have their strongest absorption lines in this range. However, molecules of moderate size can have many energy levels separated by milli- or micro-eV. Is it possible that there are electrically small molecules that absorb and emit microwaves, or lower-frequency radio waves, with atom-like efficiency? If so, what would these molecules look like?
To take the question one step further: by analogy with chemiluminescence, would it be theoretically possible to engineer a chemical reaction that produces a large amount of low-frequency radio waves from a small flask?
I am aware that some polar molecules, such as trifluoroiodomethane, have rotational spectra that extend into the microwave range. Also, hyperfine transitions are very low-energy processes. However, as far as I know, the "antenna efficiency" of these systems is typically very low. (If this is wrong, I'd appreciate being corrected.)