if you want to generate a uvw map that is the same size of your texture, do this.
1. add the uvw map modifier to your object
2. select your mapping type (probably box or planar)
3. Scroll down to the 'Bitmap Fit' button, click it and select your texture map.
This will automatically change the dimensions of your UVW map to be proportional to your texture image
For instance, if you had a marble texture that was 1080x2160, the UV mapping would be 1.0 x 2.0 (or would it be 1.0 x 0.5?).
No, the UV mapping is 1.0 x 1.0 and is independent of the dimensions of your texture map. if you changed your mapping to 1.0 x 2.0 , the resulting image would be stretched to 1080 x 4320