I want to measure the specimen size under the microscope, @@@Should I use an Ocular Micrometer ? Or Should I use a Stage Micrometer ? 
@Simply put..  

@How to Use an Ocular Micrometer 
@The magnified image of the specimen is formed on the ocular micrometer
and that the micrometer scale and the sample image can be viewed simultaneously through eyepiece lenses. At this time, the specimen size can be measured by determining the single pitch width on a micrometer scale. @The single pitch width on the micrometer scale depends on the objective lens's magnification, and it can be calculated as follows; 
Single pitch width of an ocular micrometer under the microscope = [The actual width of a single pitchnmMagnification of the objective lensn 
@Examples of measurements : 
 The figure below shows an example of measuring the size of a kidney glomerulus i) Ocular micrometer is XY11 (10mm / 100 division / 1 pitch = 100Κm) ii) Observation image with 20x objective lens 
 Applying these numbers to the formula above, the ocular micrometer's single pitch width, in this case, can be calculated as; 5.0Κm (= 100Κm 20). Since the glomerulus has 23 horizontal pitches and 22 vertical pitches, we can measure 23 x 5Κm (115Κm) in width and 22 pitches x 5Κm (110Κm) in length. 


100 Κm (1 pitch) 20 (objective) = 5 Κm X : 40 to 63 : 23 pitch x 5 Κm = 115 Κm Y : 40 to 62 : 22 pitch x 5 Κm = 110 Κm 
@How to Use an Ocular & Stage Micrometer for Calibration 
Objective lenses are allowed to have a slight magnification error. The purpose of the stage micrometer is to measure this magnification error in advance for more precise measurements. 
@Example of Magnification Error Measurement for Objective Lenses : 
 Place the micrometers in a way that the ocular and the stage micrometer
scale can be seen parallel to each other when looking through the eyepiece. You can then calculate the exact magnification from the error. 
 The figure below shows an example of an stage micrometer NOB1 (1mm/100 div/pitch=10Κm) and an ocular micrometer S11 (10mm /100 div / pitch = 100Κm) used under an objective lens at 20x. 
 If the magnification is correct, ten pitches on the stage micrometer should be equivalent to 20 pitches on the ocular micrometer. (2000 Κm = 10 Κm <one pitch of stage micrometer> x 10 pitch x 20x objective) However, in this case, the pitch reads 21, so this objective's magnification is 21x. 
 In the actual measurement, the ocular micrometer's value multiplied by
the error rate calculated 0.95 (ΰ2021) would be the more accurate measurement value. 
@How to install Ocular Micrometer 
An ocular micrometer is a glass disk with a ruled scale on the surface,
which is etched in chrome. Please insert it into the eyepiece with the print side facing the objective lens (in the image below, the lower side). 
Each eyepiece model has a different diameter to fit, and compatible micrometer sizes vary depending on the eyepiece's manufacturer and model number. Nikon and Olympus Eyepieces & Ocular Size Compatibility List * For other makers, please contact the manufacturer of your eyepiece. 
@How to check the front and back side of an Ocular Micrometer 
The scale of the ocular micrometer is printed on the glass in chrome. When
the printed side is identified, place it on the mounting bracket so that the printed side faces the objective lens side (downward).* Please also refer to the above installation instructions. This section explains how to check the front and back sides (printed side and back side). 