The smallest blood vessel is the:

Capillaries are the smallest blood vessels in the human body, typically measuring between 5 to 10 micrometers in diameter. They form a network between the arterioles and venules and are the site of the exchange of oxygen, nutrients, and waste products between the blood and tissues. This extensive network ensures that every cell in the body is in close proximity to a capillary, facilitating efficient physiological exchange processes.

References:

Gray's Anatomy

Clinical Hematology and Fundamentals of Hemostasis