Michaelides SA, et al. - This study is performed to elucidate whether the size matter, concerning the effect of position on oxygenation status in spontaneously breathing patients with unilateral effusion. Based on these, patients with large-sized effusions display the worst oxygenation when lying on the side contralateral to the effusion, while those with small-sized effusions exhibit the worst oxygenation when lying on the side ipsilateral to the effusion.