SEM can give information on pore size only on the surface. More over it won't give you any information on type and shape (narrow, cylindrical, bottle neck etc) of the pores.
Gas ads/des experiement is one of the best method for the determination of pore size which also gives the informaiton regarding pore size distribution, surface area, pore volume and much more.
Nitrogen sorption isotherms or other gas adsorption-desorption measurments can provide the curves of the mesopores size distribution. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas (SBET),The pore volume and pore size distributions were derived from the adsorption branches of isotherms by using Barrett-Joyner- Halenda (BJH) model.
If you can't use N2-sorption measurements there are two possibilities. Either SAXRD or TEM tomography.
WIth small angle XRD measurements (from 0.5 to 10 2theta) you can get the distance between the center of two adjacent pores. But this only works if your pore system is ordered, otherwise you will not see anything in the low angles of XRD.
With transmission electron microscopy (TEM) tomography you can actually see the inner pore system of the porous material. Through a line scan you can then easily estimate the pore size of your material. This of course will be a very localized measurement which shouldn't be a problem when your pore system is periodic and ordered. In other cases you will only get a general idea of the pore size.
Fact remains that the best way to determine the pore size of your materials is a N2-ads/des measurement. Then you will get a pore size distribution.
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