large particle

in radiation scattering
https://doi.org/10.1351/goldbook.L03457
A particle with dimensions comparable with the wavelength of the radiation in the medium or larger. In practice a particle must be treated as large if its largest dimension exceeds about one-twentieth of the wavelength employed.
Source:
Purple Book, 1st ed., p. 65 (http://old.iupac.org/publications/books/author/metanomski.html)