Grinding peppercorns to specific mesh sizes : AskCulinary
An adjustable manual coffee grinder like this might be your best bet for grinding spices to a specific size. To check the grind size just grind some of the spice and check it visually. If you really want to get precise about it then you might want to get a set of sieves, but personally I think getting that precise is kind of a waste unless you're making and selling spice blends.