1) ENVIROMENT POWER (default 3.0 makes hrdi looks way to bright)
It will be different for each HDRI image. Also, make sure you use that correct gamma. A .hdr or .exr needs a gamma of 1 from memory, and this changes the brightness.
2) render target RESOLUTION
3) camera focal lenght
Doen't matter - neither influence the size/position of the background. You can adjust which part of the background is at the back of the render by using a spherical projection of the environment texture, then rotating with the "Rotation" slider (Y aixs).
In example if I want to achive exectly same image as in orignal image, how to set up all parameters?
This should happen automatically with Octane. If you use a very high resolution HDR map, you will get a quality render. You can also increase the camera->aperture to blur the image if you are using a low res HDRI which is given pixellation.
Paul