Shortness of breath. It depends on what is causing it and how large it is. The more fluid the more it compresses and collapses the lung. The collapsed lung can not breathe for you. Effusions multifactorial, inflammation, heart failure, pneumonitis, cancer, post obstructive pneumonia, heart failure , kidney failure, hypoalbuminemia, granulomatosis etc. Shortness of breathatrestorexcertioninability to breathe laying flat.
SOB. It depends how large the effusion is and the cause. There could be compressive symptoms and can cause dyspnea and low oxygen level, CP as in tubercular causing pleurisy, and if infected signs of infection like fever can occur, and if large there could be pushing of the lung and trachea to the other side.