Who gets the room

Have you considered reinstating the room partition? Then DSS remains in his room and an additional bedroom is created - which your son could have if bigger than the boxroom?

Of course the children living permanently there get the larger room.
So your son is in a box room while a large empty room remains unused for 12/14 nights?

Why did you move in with a man who would think this is reasonable.
Awful behaviour.

And you sold your home for THIS????

Eh? Read the Ops posts the bigger room was divided to give he's DD & DS their own rooms. The division has been removed because of a dampness issue and the DD is no longer staying.

Circumstances have changed hence it makes perfect sense for them to reconsider who is in which room.

No that's not what happened at all 🤦🏻‍♀️

I think it’s unreasonable this wasn’t all ironed out before moving in. Why did you give the biggest available bedroom to children that were only there 2/14!? What made you think it was reasonable for a 16yo girl to share with her brother? Not really surprised she doesn’t want to stay overnight

That was always DSS's room, and then you and your DC moved in and they get to see more of your partner than his own son? Yeah don't take his room, that would be really really mean, he probably feels pushed out as it is.

Your stepson hasn't always been in the large room, which I believe was your husband's argument. He, your stepson, was in only the portion of the room that was used to make his bedroom. He wasn't sharing the room with his sister as they each had their own separate rooms after the room was divided.
Now that the dividing structure has been removed, your stepson has gained a new larger room to use for 2/14 days. Meanwhile, your son continues to use a room that's not big enough for all his stuff every single day.
That's not bloody fair. I get that your stepson probably feels that the larger room should be his and that he'd upset to be given the box room. However, you can absolutely justify doing that.
How would you ever justify to your son not giving him the larger room? How could you justify allowing your stepson to have the space of two bedrooms when your son barely has the space of one?

I think the two children who live there full time should have the bigger rooms.

The other child has a room at their own house for all their things!

I suspect your DH is already feeling like he is the visiting partner because his daughter hardly visits and his son only EOW. He may feel like he is already subsidising your DC and therefore is sticking to his guns about his DS having one of the big rooms. Do you split costs equally or do you pay more because your DC are there full time? You and your children already take up more of the house than he and his children if you get my drift and to him it may seem like you want to decrease that and shove his kid into even less space.

Have you talked to DSD about why she has stopped staying? Was there an issue with damp in her room? Would she like to stay if she had her own space? She is still very young to have no bed in her dad's house.

It pisses me off that your dh can't see beyond his 'my son your son' mentality and see that it's a case of one person who lives in the house full time not having a proper sized space anywhere in his life whereas the other one has two big rooms one he uses 12/14 and the other he uses 2/14.

Of dh can't see beyond his territorial claim and focus on plain old logistics he sounds like someone I wouldn't enjoy being around

It is very confusingly written post because you initially only referred to 3 bedrooms so it wasn't clear where you and DH slept, and said "DSD is here EOW so 2/14 days" but also "DSD no longer stays over".

Having read your updates, I think it's
Big room 1 - Your youngest DS (11) - always there
Big room 2 - Your DSS (11) - stays over 2 days every 14. Originally got the bigger room because he shared with DSS (16) when she stayed over but she no longer does
Box room - Your oldest DS (13) - always there

It's hard - if you were moving in brand new with the above situation of course it would make sense that the child who is rarely there gets the smallest room. But it's a bit different essentially kicking him out of 'his' room once he's got used to it.

Overall though it's still more unfair for your DS to have the smallest room when a big room is completely empty apart from 4/5 days every month. I'd give DSS/DH the option - either DSS keeps the room but DS can use it the days he's not here, to study, if he has a friend to stay over they can sleep there, etc or he moves to the smallest one and that's completely his own and private.

Assuming your older child won’t move back ? How old is he ? Where did he move to

A nobrainer: the kid who spends 2/14 days gets the boxroom no matter if it's DC or DSC. It's really logic, don't understand why your DH doesn't understand that?

Even if MN says you're right, I'm not sure how it helps. Your partner can still just say no, that's his DS's bedroom, he shouldn't have to move.

If DS13 is in Y9 I would leave it as it is for now and go for changing it in the summer "ready for his GCSE studies" as he will need a desk in his room. Then everyone has time to get used to the idea. But as to who gets the boxroom out of his little brother or DSS, honestly it's who wins the fight between you and your husband. Blended families are really difficult. If you could stretch to moving walls to even up the rooms, that would be ideal.

I would give DSS the option of sharing a room with DS13 or having the box room to himself. If DSS11 opts for sharing and DS13 isn't keen, then DS13 can sleep in the box room when DSS is visiting.

So if you have three bedrooms and the DC occupy them, where do you sleep? Do you have a separate dining room you can convert? Your OP is quite confusing.

Re the DC, child who lives there full time gets the bigger room. Otherwise, DSD11 has two big bedrooms, but your DS has one box room, plus he’s there full time.

You need to ask DSD how she feels. It's not ideal to leave her without a space in her dad's home. I would be concerned about this. If DSS was in a smaller partitioned room then there shouldn't really be too big an issue moving him. But maybe make a fuss of him, let him choose paint colour decor etc. Is there any way you could put the partition back up?

Whilst I can see the sense in giving the residents the bigger rooms, shifting your DSS to the box room could cause resentment. I think the feelings of the non resident child here does matter.
My resident DD had the smallest room until her older siblings (DSS and DSD) moved out. She now has DSD room.
But, we did use the older kids rooms for storing stuff like out of season clothes to make space for DD's stuff in her own room. They had no veto over that.

On so many boards about SDC it's very normal for them to not want to split time when they get to 15/16. Teens often don't actually spend a whole lot of time with their parents that age. They just want to be in their rooms. It's becomes more about the parents wanting them in their home more than the dc.

Genuinely it's often a chore at that age and it should always be about what works fir the dc

Op said the other 11/stepson can’t share with her kids

DSD still needs her own bed in her father’s house. She shouldn’t feel like a guest in what is supposed to be her own home.

I would look to fix the partition and the damp issue so you can subdivide the space again. Then you have a space for everyone and there is no issue of a bigger room sitting unused.