The compound adjective 'thumb-sized' is a modern English formation utilizing a straightforward descriptive logic. The first element, 'thumb', derives from the Old English þūma, a word of ancient Proto-Germanic lineage referring to the distinctive digit. The second element, 'sized', originates from the legal and assessment term 'assize', which evolved in Old French to denote a fixed amount or magnitude. By combining the concrete noun with the adjectival suffix '-sized', the term creates a universally understood, informal unit of measurement, leveraging the common human experience of the thumb's approximate dimensions to convey a quick, relatable sense of scale.

Structure: thumb (body part) + sized (having a size) - thumb: From Old English þūma, of Proto-Germanic origin, meaning the short, thick first digit of the human hand. It functions as the noun base specifying the object of comparison. - sized: From the noun size (from Old French sise, ultimately from Latin assidere 'to sit to, assess') + the past participle suffix -ed. It functions as an adjective-forming suffix meaning 'having a specified size'.