the compound adjective "walnut-sized" is a modern english formation following a transparent descriptive pattern. the first element, "walnut", originates from old english, where it literally meant "foreign nut", likely distinguishing it from native hazelnuts. the second element, "sized", derives from the noun "size", which entered english from old french, ultimately tracing back to a latin root for "a statute" or "fixed amount". the combination employs a straightforward analogy, using a common, familiar object (the walnut) as a standard unit of comparison to convey a specific, roughly spherical volume or dimension, a common method in everyday measurement description.

Structure: walnut + sized walnut (from old english wealhhnutu, meaning "foreign nut", via the germanic root for "welsh/foreign" + "nut"); sized (from middle english sised, from size + the past participle suffix -ed, indicating "having a specified size").