The choice of the most appropriate method depends on the application of the porous solid, its chemical and physical nature and the range of pore size.

The most commonly used methods are as follows:

a) mercury porosimetry, where the pores are filled with mercury under pressure. This method is suitable for many materials with pores in the appropriate diameter range of 0,004 μm to 400 μm.

b) meso- and macropore analysis by gas adsorption, where the pores are characterized by adsorbing a gas, such as nitrogen, at liquid nitrogen temperature. The method is used for pores in the approximate diameter range of 0,002 μm to 0,1 μm (2 nm to 100 nm).

c) micropore analysis by gas adsorption, where the pores are characterized by adsorbing a gas, such as nitrogen, at liquid nitrogen temperature. The method is used for pores in the approximate diameter range of 0,4 nm to 2 nm. 

Above is according to ISO 15901..

But if you asking about the standard  operating Procedure and how to prepare you sample..you can check out the attached file... the idea of measuring is rely on adsorption technique  and there are many recent articles discussed  this technique deeply..

The choice of the most appropriate method depends on the application of the porous solid, its chemical and physical nature and the range of pore size.

The most commonly used methods are as follows:

a) mercury porosimetry, where the pores are filled with mercury under pressure. This method is suitable for many materials with pores in the appropriate diameter range of 0,004 μm to 400 μm.

b) meso- and macropore analysis by gas adsorption, where the pores are characterized by adsorbing a gas, such as nitrogen, at liquid nitrogen temperature. The method is used for pores in the approximate diameter range of 0,002 μm to 0,1 μm (2 nm to 100 nm).

c) micropore analysis by gas adsorption, where the pores are characterized by adsorbing a gas, such as nitrogen, at liquid nitrogen temperature. The method is used for pores in the approximate diameter range of 0,4 nm to 2 nm. 

Above is according to ISO 15901..

But if you asking about the standard  operating Procedure and how to prepare you sample..you can check out the attached file... the idea of measuring is rely on adsorption technique  and there are many recent articles discussed  this technique deeply..

thanks mr mohammed obeid. i found that a few researchers use scanning electron microscope and optical microscope to measure the pore size. In my case, the sample is porous or cellular metal.

Indeed I used SEM to measure the pore geometry, overlapping and density (porosity) of x-ray filters made on plastic (PET) track-etched membranes. Our filters were metal-coated on one side which can fill the pore. In this case , the pores cannot be considered as cylindrical. I needed to make a cross section to correctly define the inner pore size (orientation as well).