3D plane size of canvas container

I would like to know what us the conversion between pixels and three js units. I know that it is in meters and we can convert from pixels to meters, but this does not give the actual size for a planegeometry that we want to have the actual size of the canvas component, for example in a regular laptop the width and height off the plane geometry in order to cover the entire window is around 15 units. I have tried adjusting one side and then multiplying by the container pixel ratio but this is not always accurate. Thank you