Hi Linh,
There are several methods described and to be honest I think they are all rubbish.
If you measure the length of the right lobe it is super variable with some people having a Riedels lobe, others having a big left liver and short right lobe so I think the natural variation destroys the test. Once again I look more globally. An enlarged liver will have blunt edges, will hang down well over the pancreas and likely cover the right kidney. CT is a much better way to assess liver size as it gives a global perspective.
We take a measurement in the mid clavicular plane and I reckon it is a bit of a waste of time compared to a subjective assessment.
Steve