we have a "cosy cottage" style house where we have dormer windows in the 2 bedrooms upstairs with timber cathedral ceilings. Causing some problems with having wardrobes for clothes storage. In our spare bedroom downstairs we solved this problem by installing a built in wardrobe with mirrored sliding doors. This room is right beside the bathroom so it works really well as a dressing room.

janbhan my goodness he's a worry wart lol I'm lucky our business is downstairs so whenever we need toilet paper , tissues, toothpaste we pop them on our account and bring them up to use..no more having to pop to the shops..I still buy toothpaste at the shops though..I find the pumps are less messier than the tubes..hubby can't get them in so we use tubes when we are out ..

You could add a chaise lounge or a comfy chair and table for a reading nook or an office area if you need one, a computer area but whatever you do don't over clutter it keep it sleek and sexy and or a comfy chair as suggested with a little side table for reading, I also like the idea of a place to put coats, shoes etc if you must enclose it. Maybe even a small wine bar? with a couple of stools. I decided to google is and will give you a link to many pics to give you more choices. https://www.google.com.au/search?q=pics+of+what+to+do+with+space+behind+floating+stairs&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjFy-n9p4vZAhUEF5QKHSvhAOMQ7AkISg&biw=1280&bih=893

Two issues -- The middle size looks 'about right' and approx 40cm diameter ( approx 2/3rds the size of the larger one ) . But I don't think it suits many modern styles -- a beach inspired place with say white woven wallpaper or seagrass style flooring or some distressed furniture would be complemented by this , but a modern build with say painted walls doesn't do anything with this style IMO . Go classier rather than laid-back would be my advice , and that would depend on the decor . And thinking about things , one 'trick' I used in a commercial building I own was 5 polished copper shades up an L shaped stairway -- the ceiling was flat , so nearly 5 metres 'up' at the base , and I used 28cm ones with the biggest round clear LED bulbs ( 8Watt ) I could get , all hung on that oldstyle '1940's iron' type cord , so they were all 2.6metres above the stair below -- it created that 'going up' visual effect . One large light ( especially say 4 metres above the deck ) won't give that effect .