You can give individual objects their own UV if you want as well as their own textures. The major factor is the resolution of the image that you fit in the 0-1 space.

For games, 256x256, 512x512, etc. Movies and such, you can go much higher. You usually want the resolution of the texture to be in relation to the overall size of the object. A chair's texture would usually be a smaller resolution then, say, a building, except for the case where the texture repeats.

You don't necessarily have to keep the UVs in the 0-1 space, but keep in mind that the texture tiles in the UV Texture editor outside of that space.