The 21 centimeter hydrogen line originates from the hyperfine transition of neutral hydrogen. Are there any bigger wavelengths that originate from this method? Radio emissions from rotating stellar objects are not part of this category; clarifying in case this is brought up.
Any ion that captures an electron has the capability of forming a hydrogenic atom where you can have transitions between energy levels with very large principle quantum number. The conditions to make these transitions visible are a low density plasma to avoid collisional (de)excitation.
For example Peters et al. (2011) discuss "decametre" wavelength transitions, and some of these have been detected, mainly associated with carbon ions (e.g. the C631$\alpha$ radio recombination line at 26 MHz $=$ a wavelength of 11.5 m).