The maximum size of main carry-on bags will remain unchanged, but smaller second bags such as handbags will be capped at 40x30x15cm, the airline said.
It said there had been ?an increase in customers travelling with hand baggage that exceeds their allowance? in recent months.
The change will come into force ?in the coming weeks?, the airline added.
Currently, passengers can board flights carrying one bag of up to 56x45x25cm, weighing up to 23kg, plus another bag up to 45x36x20cm.
A spokesman said the new guidelines ? which apply to all customers ? meant the latter changing ?ever so slightly?.
He said overloading of hand baggage caused delays as staff removed bags from the cabin.
Last month, a teenager from Scotland collapsed with heat exhaustion after wearing 12 layers of clothing to avoid paying a ?45 extra baggage fee.
James McElvar threw up and was given oxygen during the Easyjet flight from Stansted to Glasgow.