If you think next as absolutely minimums for standing and/or lie down model: 1,5 meters from background to model, place needed from background stands to wall atleast 0,5 meter, 2 to 3 meters from model to camera that is you, and atleast one meter behind you so you can to move, that is 5 to 6 meters lenght. And I don't know if you can put hair backlight brhind and above models head, mayby do, but... 2 meters from each wall to light stands, and 2 meters between lights, that is 6 meters. And let say height of model atleast 1,80 meters (some will be shorter, but same models can be ove 2 meters high), so when you think that you will have to place lights above models head, space needed fior umbrellas/softboxes, theigh needed for hair backlight, that is 3,5 meters. So, absolutely minimum would be 6x6x3,5 meters. And that would be enough for sitting or 3/4 heigh (american plan). Maybe you can shoot one standing/laying down model with average heigh up to 1,80 meters. Best dimension for comfortable work of anything except lager people groups/cars/and bigger subjects would be 10 meters lenght, 8 meters width, and 4 meters and above heigh. In that room you can put also some table/chairs/couch for resting and make up/hair dressing, and paravan for models cloths changing.
That would be my dream studio, 10x8x4 meters.
Windows: As large as you can on north side if you can. Wall colours: white. But, make such arrangement that you can in any time darker the room (place black material over the windows), and that you can place dark (black) material behind background and on side walls. Sometimes you would need to minimize light bouncing from walls, that is reason.