I was not too long ago I was going to build one in an existing (large) factory building.
If I can recall, it was going to be 6 meters wide by 4 meters high and 17 meters long, this would give me enough space for a toilet, shower and make up area, a small kitchen and viewing area, this would leave me with a 10 x 6 meter shooting area.
I also had use of another 1200 m2 of factory space for cars, bikes and stuff if needed.
The studio could could have been built for just under 3000 euros, I would do most of the work though.
Nice size, most would not be that large I would think
I`m lucky in that i now use parents old garage its 300sqr metres !!!!lovely and it allowed me to build an infinity wall, extravagance i know but so wonderful.
In reality if it used full time as a working studio i would say minimum of 200 sqr metres, its amazing how it just grows.