Customers with existing bookings from 10 February will not be charged extra.
EasyJet is to start charging passengers for hand luggage that does not fit underneath the seat in front of them.
The Luton-based airline has announced that passengers travelling on its standard fares will, from 10 February, face paying an extra fee for cabin bags that have to be stored in an overhead locker.
It's a change from the current policy, which allows customers to fly with cabin bags measuring up to 56 x 45 x 25cm, allowing them to place small wheelie suitcases into the storage above.
Soon carrying a bag of such size will command an additional cost starting at £7.99, which will also allow travellers to choose a seat closer to the front of the plane or with extra leg space.
Five test bookings by the PA news agency for flights from Gatwick Airport on 11 February 2021 found prices between £17.49 and £21.49, adding up to 45% to basic fares.
Those with the more expensive FLEXI fare - or with membership to the company's frequent flyer scheme - will be allowed to continue taking one small and one large item with them, subject to space.
EasyJet, which has described its size restrictions as "among the most generous for air travel", said the new rules were designed to "improve boarding and punctuality" on its planes.
Customers with existing bookings for travel from 10 February next year, who do not want to upgrade their tickets, will be able to check in a larger cabin bag without cost.
There will still be no weight limit on cabin bags - only size.
EasyJet chief commercial and customer officer Robert Carey said: "Punctuality is important to our customers and we know that, if they have their bags placed into the hold at the gate due to the limited space onboard, this can cause flight delays and it can be frustrating for them too.
"Our new policy will improve boarding and punctuality for everyone, as well as give our customers certainty of what they will have with them onboard.
"For families, we'll continue to provide a number of options to allow them to bring additional items free of charge, helping keep travel easy and affordable for families when they fly with us."