I know the common workflow for texturing interiors is using tileble textures with combination of decals/alphas, but hypotetically speaking, if you would HAVE TO use one texture for one wall of squarish living-room type of an interior (so four big textures in total), what texture size would you use for best quality in…
I wouldn't. Unless I was using Rage technology. http://wiki.polycount.com/wiki/MultiTexture#Megatexture For all other games, I'd use tiling textures and texture blending, period.