This is a useful piece of medical trivia to know.
As you're aware, a pleural effusion is defined as an increased amount (usually more than about 10-20 ml) of pleural fluid in the pleural space. It is caused by an imbalance in the usually balanced rates of pleural fluid formation and absorption; in other words, either too much pleural fluid is being formed, or too little is being reabsorbed. The list of possible causes is very long, but the usual first step is to differentiate between an exudate and a transudate.
But enough background - let's get back to the question. Astonishingly, even though 25 mls is enough to count as an effusion, chest percussion can only detect pleural effusions of greater than about 500 ml!
Chest X-rays are better, but the sensitivity depends heavily on how the patient is placed. With the patient sitting or standing (an 'erect' chest X-ray), the first sign of a pleural effusion - blunting of the costophrenic angle - occurs once 200-300 mls of pleural fluid has accumulated. With the patient lying on his/her side (a 'lateral decubitus' chest X-ray), as little as 50 mls of fluid may be detectable. Below is a large left-sided (hence on the right as you look at it) pleural effusion in an erect chest X-ray.
As an aside, it is believed that at least a litre of fluid must be present for the fluid to push aside other chest structures, a phenomenon known as 'mediastinal shift'.