You aren’t being forced to stay there though - if you moved somewhere else then you’d have a better chance (but by no means guaranteed) of getting a refund through the chargeback process.
You’re choosing to stay there, and asking for a refund. That won’t happen.
Complain to the hotel, write a bad review and get on with it. We all take Ls and this time you might have to take it.
If the hotel was safe, clean and had everything you need (apart from space apparently), I don’t think it’s that much of an issue.
Like someone mentioned earlier, talk to the hotel about it and see what they can offer.
I would say, as I deal with a lot of chargebacks of a very similar nature directly, there is from my experience very little chance of success for a misselling claim. It would stand more chance if they could provide photos of the room that was advertised as being very different to the room received. Even then, it would likely be a small partial refund (20% at most) rather than the full thing.
As we say to many people the absolute best chance of a resolution is to speak to the hotel themselves for a suitable goodwill gesture. I’d never advise actively leaving accommodation without the hotel or host’s agreement unless you have near airtight evidence of why it was impossible for you to stay there.
Thank you very much for all your responses, next time I will use other booking provider and will book just for couple of days instead of weeks, also will read hotel review before I purchase and use credit card.
I haven’t seen the room listing or the the regulations but 16.8 to 24 is a big difference in size, trivial to some but not if you have a wheelchair or buggy and have no manoeuvrability.
Accessibility is something that should really be noted separately though, or enquired about. If you have such requirements it wouldn’t be a good idea to just rely on the little bit of text that gives square footage. Again, pictures should also generally give a good idea of the room size.
As a general principle of something is very important to you it’s best to ask about it and check before booking.
By all means write a quick email to express your disappointment but beyond that, for what the difference is and how long you spend in a hotel room on holiday I’d just let it go.
Getting stressed about it, arguing your case on here, with staff, with booking.com and in general going out of your way to buy tape measures and the likes just isn’t worth it.
Enjoy your holiday, don’t let it ruin the experience. Or your memories of it, if you’re back home now.
A chargeback is not the way to go for this, but I’m surprised at some of the comments aimed at the OP. If I book a 26sq m room I expect to get a 26sq m room. They are saying they got a (just under) 17 sq m room - that is over a third smaller than advertised and would be VERY noticeable (I assume that is why a tape measure was procured - to check). It is absolutely worthy of complaint (not a chargeback).
Everyone here would complain if they ordered 100 bedding plants but only got 66 delivered? This is the same - not getting what was paid for. The booking agent should have dealt with this properly and at the very least given some credit IMO.
As above I think this is the wrong sort of comparison/way of looking at it for a number of reasons.
Personally if the room looked like what was pictured when booked I almost definitely wouldn’t notice the error in the square footage. If I somehow noticed I might send a polite email to booking.com so they can update it but I’d still feel I got what I paid for.
However, if it did irk me slightly I guess I’d point it out to the hotel to see if there’s something they can do, e.g provide a bigger room, which is what I think everyone has suggested
Booking.com is a third party booking site/aggregator, isn’t it?
Entirely possible, then, that the rooms of the standard OP booked were overbooked, so they’ve stuck OP in an alternative room and hoped he wouldn’t notice. So I can understand the upset. For a slightly extreme fictional example, the first season of White Lotus had someone furious that they’d booked the best room in the hotel and got second best 
However, as noted, chargeback isn’t going to be the best way for a resolution. Being nice to reception might be able to see them find an alternative room, or knock some money off the bill. Failing that, suggesting a scathing TripAdvisor review will be left.
the whole storyline was about how his childish tantrums over a small issue ruined his honeymoon
In my experience, photos of hotel rooms when you book show the biggest room of that room category. They are not always all identical rooms in shape and size. Even Travelodge and Premier Inn have variations in room sizes within room types within the same building.
The size of the bedroom is just a small part of what you are paying for in a hotel
It isn’t a small part - it is a large chunk. The room is the entire basis of choosing a hotel. You don’t choose a hotel based on how efficient reception staff are. Some may choose a hotel based on how good the restaurant is (for example), but the room is still the basis of the booking.
Personally if the room looked like what was pictured when booked I almost definitely wouldn’t notice the error in the square footage.
If it was a small amount, sure. But we are talking an entire third in this case.
Honestly other than @lpoolrob I’m surprised by the number of folk who wouldn’t care they were short changed on a hotel room. I’d absolutely complain if that was the case. Often there are quite large increases in costs for what is essentially only a few metres square extra. Pictures are absolutely not one to go by - I stay in hotels a LOT and when booking they are almost always “representative” and not the actual rooms you get.
Chargeback isn’t the way to go though. It’s nothing to do with Monzo. Would be a complaint to the hotel. Might or might not get anywhere with it.
I wouldn’t let it ruin my holiday mind you, not worth getting too worked up over it.
But absolutely a complaint. You’re not getting what you paid for.
5.1m x 5.1m vs 4.1m x 4.1m
I don’t think it’s a huge deal, worth a moan at the front desk, sure, but not going to buy a tape measure, complaining to the bank, registering on a forum to complain about it levels of bad. Especially not when on holiday!
Those measurements make it seem a smaller difference than it is. It would be very unusual to have a square hotel room. I won’t do the maths but when you assume it is a rectangular hotel room roughly twice as long as wide, the different would be significantly more noticeable.
But I agree, something to sort out when you get home (if the hotel won’t move you to the room size which was booked), and not something for a bank to sort out.
Honestly just wouldn’t bother me and I rarely if ever complain while on holiday because I’m busy getting drunk or doing other things. I just sling my bag in the room and go. Obviously if I’d booked somewhere with a desk or a bath or a balcony and it didn’t have one, that would be an issue, but a little extra empty floor space? Not even worth a thought in my mind.
Still, if it bothers people I think we are all agreed it’s best just to speak to the hotel and see if anything can be done.
If I, on my honeymoon, went out and bought a tape measure because I thought the room looked a bit small, my wife would immediately divorce me.
And she’d be right to.
It would bother me but possibly more if there were cheaper rooms at that same size. In other words if I pay for a room and get a cheaper room and expected to put up with it.
Tbh I wouldn’t notice if it weren’t a lot, but if I did notice it, I’d certainly complain. Just likely after the fact and with no anticipation of getting anything.
they were there for weeks not a 5 day holiday
Maybe I just haven’t noticed but I don’t feel like I’ve ever seen a hotel where rooms were priced differently solely on square footage. More expensive rooms have always had something in the way of extra amenities, better spec or location listed?