If you are not sure about how to calculate the step size,we highly recommend Johan’s page

http://extreme-macro.co.uk/focus-stacking/#calculator

Also some value may help you to set length of step( 1 mm =1000 micron )

Such as Canon MP-E65

If you use microscope objective lens

from

http://www.microscopyu.com/articles/formulas/formulasfielddepth.html