The compound adjective "thumb-size" is a transparent modern English formation. Its first element, "thumb," has a direct lineage from Proto-Germanic þūmô-, a word for the thick first digit of the hand. The second element, "size," entered English via French legal terminology concerning the setting of standards, which evolved to mean fixed dimensions. The compound logically combines these two concepts, using a familiar body part as a universal, informal unit of measurement to convey a small, approximate scale, bypassing the need for precise metric or imperial units. This follows a common pattern in English for creating accessible descriptive terms, such as "palm-size" or "fist-size."

Structure: thumb (body part) + size (dimension) - thumb: From Old English þūma, of Proto-Germanic origin. Functions as the base noun denoting the specific body part. - size: From Old French sise, a shortening of assise ("assize, regulation"), ultimately from Latin assidēre ("to sit beside"). Functions as a noun indicating dimensions or magnitude.