This was only a minor problem, prior to the advent of battery operated sampling equipment. Too small a pore size would only result in a too rapid, plugging or blinding, of the filter so that the requisite quantity of air could not be drawn through it. With battery operated equipment, too small a pore size would lead to high power consumption, premature shutdown and with unregulated equipment, and incorrect flow rate. If a range of pore sizes have been recommend, use the largest. Consult the manufacturer of your sampling equipment for the correct type and pore size to use.
Many manufacturers provide application notes regarding correct filter applications.
Avoid small diameter filters and small pore size membranes. The physics of the filtration of particulate material from a gas stream is complex. Several mechanisms are acting in concert or opposition. Those desiring an understanding of filtration theory should consult the following source, which also contains a data table. See Chapter 10 starting on page 179.