'pocket-sized' originates from modern English, specifically a compound of 'pocket' and the adjective-forming element 'sized' (from 'size'), where 'pocket' meant 'small bag' and 'size' related to 'measure' or 'dimension'.

'pocket' changed from Old North French 'poket' (a diminutive of 'poke'/'poche' meaning 'bag') into Middle English 'poket' and eventually the modern English 'pocket'. 'size' entered English from Old French 'size' (meaning an extent or fixed amount), and the adjectival form 'sized' developed by adding '-ed' to 'size', with compounds like 'pocket-sized' appearing in the 19th century.

Initially 'pocket' referred specifically to a small bag; over time, when combined with 'sized' the compound came to mean 'small enough to fit in a pocket' or 'designed to be conveniently small', which is its current usage.