I can say from personal experience.
I commute weekly between EDIand STN. To make it as easy as possible I always travel with a single piece of cabin baggage. In testament to the quality of the product, I had used the same Travelpro Platinum 3 bag for 12 years on numerous airlines across the UK and the rest of world. It fits absolutely perfectly in to the overhead lockers (the correct way, wheels in, not sideways) as it was designed to do.
One Friday at STN I was first in the Speedy Boarding line and was approached by one of the Menzies gate staff who said my bag was too big. I explained that I've used it for many years and it fits perfectly in the overhead and that I had had no issues with taking it onboard on previous flights. She told me to put it in the sizer which I did. It would not go right in as the wheels, were catching the edge by around 1cm. The main part of the bag would have fitted without issue had the wheels not protruded. I was then informed that unless I paid £45 I would not be able to travel.
I pointed out to staff member that it was only the wheels preventing it from fitting but this was a complete waste of my time as she was not willing to apply any logic or discretion.
I paid my £45 and the bag was tagged and I was told to take it to the bottom of the steps.
On boarding I carried my bag to the aircraft as usual and as I have done many time before, placed it in the overhead locker where it fitted perfectly. In the end I paid £45 to take my bag in to the cabin.
I now have a new TravelPro bag that does fit in the sizers and I now use that.
EZY have changed the policy slightly for certain groups of customers:-
Cabin bag changes from Thursday 19th March 2015
All customers can bring ONE cabin bag up to the maximum size of 56 x 45 x 25cm including handles and wheels. Overhead locker space is limited on board the aircraft and on busy flights your cabin bag may have to go in the hold.
If you’re an easyJet Plus cardholder, FLEXI fare or Upfront or Extra Legroom customers then you can bring ONE additional small under seat bag (handbag/Laptop) with a maximum size of 45 x 36 x 20cm which will need to be placed under the seat in front. In order to secure your overhead locker space we recommend that you use easyJet Plus Speedy Boarding to be amongst the first to board the aircraft.