How large can textures be for buildings in a city?

I’m making a city for a responsive game (for pc and phone), and I wonder what would be best texture size for each building if gamer is going to enter the buildings through their door. What wouldn’t be a problem for performance? What would you suggest 512px², 1024px², 2024px²?

In short - 2048x2048 (whenever possible, use lower.) And for mobile skip using PBR map props unless they are absolutely vital - PBR textures take away space that could be used by color textures, while giving little in return on smaller screens.

If the buildings are enterable - separate them at least into floors.

Do not use one texture for the entire building - unless it’s, in fact, covered in not many details. If you can pack all the data into 2048 texture - go for it. If you can’t - use separate textures and materials for different floors / surfaces / objects. Packing everything into a single texture usually works only for low-poly / PSX style kind of games.