Could we see some more pics of the room?

Dear aauj, You can build that whatever size to suit your room. It's a free standing structure. That photo is of something that someone has had made, you cannot to my knowledge purchase something like that. The only thing slightly similar would be a bookcase on wheels that is often used to divide a space, but that is moveable to suit.

Ah ok. Well we have always gone off the opening size of the hole where the fire will go then deduct the space you need to leave all around it and that gives you the max size then of the wood burner you can have. I would advise speaking to a shop that sells them for advice, or your fitter if you have one already. Basically the opening will be set width ways as to what you can have (presuming it’s all closed off at the moment so you can’t measure it?) then I think it’s something like 100mm either side and slightly less at the back you have to allow (don’t quote me!) The depth of the space doesn’t matter so much as if it’s narrow then you can get a wood burner that curves outwards into the room which actually gives off more heat as it’s not lost up the chimney! So firstly, find out roughly the size of the opening you will have. Then look at the sizes of the burners with the wattage you want. We’ve had a couple fitted and getting another one done and my husband fitted them for a while with a local company so that’s how he does it

I'd probably go with 3m x 1.m to leave an equal gap of 1m all around, and that would be the largest i'd go. It's not an overly large room and I think the roof lantern will dominate it rather than add to it if too large. Also, look at what's commercially available as off the shelf purchases or you'll pay a lot more for custom sizes. Here's an interesting article Sterling Build lantern Sizes 3m x 1m is a standard size starting at £1109. Here's a link to the products and prices Sterling Build Roof Lanterns