The degree of coarseness or fineness of a powder is expressed by reference to the nominal mesh aperture size of the sieves used for measuring the size of the powders. For practical reasons, the use of sieves, for measuring powder fineness for most pharmaceutical purposes, is convenient but devices other than sieves must be employed for the measurement of particles less than 100 mm in nominal size.
The following terms are used in the description of powders.
Coarse powder:- A powder all the particles of which pass through a sieve with a nominal mesh aperture of 1,700 mm and not more than 40% by weight through a sieve with a nominal mesh aperture of 355mm.
Moderately coarse powder:- A powder all the particles of which pass through a sieve with nominal mesh aperture of 710mm and not more than 40% by weight through a sieve with a nominal mesh aperture of 250mm.
Moderately fine powder:- A powder all the particles of which pass through a sieve with a nominal mesh aperture of 355mm and not more than 40% by weight through a sieve with a nominal mesh aperture of 180mm.
Fine powder:- A powder all the particles of which pass through a sieve with nominal mesh aperture of 180 mm and not more than 40% by weight pass through a sieve with a nominal mesh aperture of 125 mm.
Very fine powder:- A powder all the particles of which pass through a sieve with a nominal mesh aperture of 125 mm and not more than 40% by weight pass through a sieve with a nominal mesh aperture of 45 mm.
Microfine powder:- A powder of which not less than 90% by weight of the particles pass through a sieve with a nominal mesh aperture of 45mm.
Superfine powder:- A powder of which not less than 90% by number of the particles are less than 10mm in size.
When the fineness of the powder is described by means of a number, it is intended that all the particles of the powder shall pass through a sieve of which the nominal mesh aperture, in
mm, is equal to that number.
When a batch of a vegetable drug is being ground and shifted, no portion of the drug shall be rejected, but it is permissible, except in the case of assays, to withhold the final tailings, if an approximately equal number of tailings from a preceding batch of the same drug has been added before grinding.
 
Method

• For coarse and moderately coarse powders: – Place 25 to 100 g of the powder being examined upon the appropriate sieve having a close-fitting receiving pan and cover. Shake the sieve in a rotary horizontal direction and vertically by tapping on a hard surface for not less 20 minutes or until shifting is practically complete. Weigh accurately the amount remaining on the sieve and in the receiving pan.

• For fine and very fine powders: – Proceed as described under Coarse and moderately coarse powders except that

(a) the test sample should not exceed 25 g,
(b) the sieve is to be shaken for not less than 30 minutes or until sifting is practically     complete, and
(c) the particles passing through a sieve of nominal mesh aperture of 45 mm are determined by suitable microscopic examination.

• For micro fine and superfine powders:-Proceed as described under Fine and very fine powders except that the particles passing through a sieve of nominal mesh aperture of 45 µm (for fine powders.) or those less than 10 mm in size are determined by suitable microscopic examination.

With oily and other powders which tend to clog the openings, carefully brush the screen at intervals during sifting. Break up any lump that may form. A mechanical sieve shaker which reproduces the circular and tapping motion given to sieves in hand sifting but has a uniform mechanical action may be employed.
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Reference links
http://www.usp.org/sites/default/files/usp/document/harmonization/gen chapter/m99610_811_powder_fineness.pdf
http://www.uspbpep.com/bp2008/data/839.asp