My studio is a tiny 2.4m wide x 2.9m length. I record mainly acoustic guitar, occasionally electric guitar miking up the THR10, nothing like EDM or dance with heavy bass requirements.
I also use my studio for teaching. It is not acoustically treated at present, although I am about to start placing a few bass traps as I get some resonance boom in the 100-200hz range. In fact just sitting at my desk and talking (nothing coming out monitors) whilst recording guitar lessons on that Logitech webcam attached to my PC monitor I can hear a boomy resonance at times when I play back the recordings. Anyway this post is about monitors, not acoustic treatment. But I can't go overboard on the latter as I'll have no space left, and couldn't justify it anyhow.
So.. my Tannoy Reveal monitors are already too big for this space and my desk (140 x 60com) I think(?), and whilst they are angled their nearest edge to the rear wall is just 3cm away, not great with rear ported monitors in this confined space. I am about to install a 10cm Rockwool bass trap panel on the wall behind my PC monitor & speakers so will have even less space, even if I pull the desk away from the wall.
My question is what size monitors to go for, considering the above room and my needs? A pair of used Yamaha HS5s locally are tempting me which in terms of footprint is 4cm less depth and width, 6cm smaller in height. Not bad space saving but maybe I should go all the way to a very small monitor like the IK Multimedia ILoud Micro?
Budget couple of hundred or so, happy for used. What do you guys think?

Comments
No need to use depth to treat behind the monitors, maybe a small bit above them to cut down on reflections even more.
I would not waste money changing your monitors. Deal with the room first - or find a more suitable room or position within that room.
And the monitors absolutely make a difference if that's what you can actually change, and monitoring at low levels works quite well if you're not trying to get perfect polished low end.
What is more of an issue in small rooms is when bass (low long waves take a few feet to develop) hit a solid back wall and bounce back toward you and cancel out approaching waves, causing weird varying frequency dependent cancellation at different distances from the listening position. So I'd recommend banging bass traps behind you to start with and thin side panels for early high reflections.
On the window a wooden slat blind could be a decent diffuser.
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If not then get what you can afford and don't worry.
If you are releasing it then just do the final mix in a proper studio, and/or have someone like me mix it for you (in my studio).
On room modes etc:
You have to measure the room and find out where the problems are, which isn't hard but for some reason people are hesitant to do it.
Follow this: https://gikacoustics.co.uk/room-eq-wizard-tutorial/
Re smaller cones: in theory the smaller the monitor the lighter the bass, the less you have to worry about bass interacting in unhelpful ways.
The downside is you aren't hearing the fun bit of audio- the bass.
If you can't hear it, how can you mix it?
There are a load of best practices when it comes to room design and approaching a mix.
Learning those and then figuring out which ones you pay attention to and which ones you ignore is very much a personal thing.
For instance, I live in a rural area and I wanted a good sounding room but I don't give a fuck about isolation.
So I have a great sounding room with almost no sound proofing.
If I lived in a flat above other people then I'd make different choices.
Products like DAD SPQ, Trinnov (or at the cheaper end, Sonarworks) are very helpful at fine tuning if you don't want to or can't spend a lot on room acoustics. They don't make a bad room turn into a world class facility, but no one should expect them too. In my space SPQ and the Trinnov give me a few extra percent of accuracy in a room that is already reasonably flat.
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re GIK Acoustics, have now been through the room quote/consultation process with them, the bill comes to about 750 quid just to make some kind of improvement- corner bass traps, ceiling and right hand wall panels (and their view was all or nothing). In honesty I cant justify that at the moment, though if circumstances do change I would go back to them.
Interesting about the speaker cone size, it sounds like it might actually help to have smaller monitors to help reduce boom --when I am simply playing music in my studio or teaching online for example - but when I do mix things go for some open backed headphones as @Musicwolf suggested (mine are closed back). As you point out, this isn't commercial release stuff after all.
Thanks for your input @octatonic
You can trust GIK- yes they are trying to sell you stuff but they don't suggest things that aren't needed.
I spent 3.5k with them on my current room and they were sensible about their suggestions which mostly correlated with the acoustician I used.
£750 is a lot but they are reusable and a quality product.
Also look at Sonarworks for room correction.
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This could be a function of the amp I've been using with the Tannoys (a Denon hi-fi amp set to flat), but I don't think so.