I have read that the average size of a capillary is about 8 micrometers. How is it possible that the 15 micrometer or so monocytes in blood do not block these vessels?
They aren't completely rigid and can change shape to squeeze through (see Downey et al).
If they are activated, monocytes can get stuck in capillaries and block them, which contributes to poor circulation following reperfusion after an ischemic blockage (see Engler et al).
Downey, G. P., Doherty, D. E., Schwab 3rd, B., Elson, E. L., Henson, P. M., & Worthen, G. S. (1990). Retention of leukocytes in capillaries: role of cell size and deformability. Journal of applied physiology, 69(5), 1767-1778.
Engler, R. L., Schmid-Schönbein, G. W., & Pavelec, R. S. (1983). Leukocyte capillary plugging in myocardial ischemia and reperfusion in the dog. The American journal of pathology, 111(1), 98.