'image-size' is a modern English compound formed from 'image' + 'size'. 'image' originates from Latin, specifically the word 'imago', where 'imago' meant 'likeness, picture'. 'size' originates from Middle English 'sise' (from Old French), a word used for a measure or extent.

'image' entered English via Old French 'image' and Middle English 'ymage', eventually becoming modern English 'image'. 'size' developed in Middle English as 'sise'/'size', originally referring to a fixed amount or measure and later generalizing to 'extent' or 'magnitude'. The two elements were combined in modern technical usage to express the magnitude or dimensions of an image.

Initially, the components separately meant 'picture' (image) and 'measure/extent' (size); over time the compound came to be used specifically in technical contexts to mean an image's dimensions or storage magnitude (pixel dimensions, file size, or printed size).