'equal-sized' originates from Modern English, specifically the combination of the adjective 'equal' and the adjective-forming past participle 'sized', where 'equal' meant 'same in measure or amount' and 'size' meant 'magnitude or extent'.

'equal-sized' changed from separate words in Middle and Early Modern English: 'equal' (Middle English 'equel' from Old French 'egal', ultimately from Latin 'aequalis') and 'size' (Middle English 'sise' from Old French 'sise'), and was later combined in Modern English as the compound adjective 'equal-sized'.

Initially, the components referred separately to 'level/even' ('equal') and 'magnitude' ('size'), but when compounded as 'equal-sized' in Modern English the meaning became the specific descriptive sense 'having the same size'.