Final answer:
The left lateral decubitus chest radiograph is the preferred projection to visualize a small left-sided pleural effusion, as it allows the effusion to layer and be detected clearly.
Step-by-step explanation:
To best demonstrate the extent of a small left-sided pleural effusion, the most appropriate projection is the left lateral decubitus chest radiograph. When the patient is positioned on their left side, any effusion present will layer along the gravitational gradient, allowing it to be visualized clearly as it separates from the lung tissue. The left lateral decubitus position is superior to the PA chest, left lateral chest, and right lateral decubitus chest for identifying a left-sided pleural effusion because it provides a better opportunity for the effusion to move and form a visible layer along the dependant portions of the pleural cavity.