'full-sized' originates from Modern English as a compound of the Old English adjective 'full' (Old English 'full'), where 'full' meant 'filled' or 'complete', and the noun 'size' (from Old French 'sise' or 'sis', ultimately via Vulgar Latin from a root like 'situs'), where that root referred to 'extent' or 'position'.

'full-sized' developed from the phrase 'full size' in Middle and Early Modern English and later stabilized as the hyphenated adjective 'full-sized' in Modern English to describe something that possesses full size.

Initially it simply described having 'the full measurement or extent'; over time it became more specifically used to mean 'of standard/normal (not miniature) dimensions' in everyday usage.