Examining the results of a large set of quasi-static cyclic tests on unreinforced masonry (URM) walls showed that the drift capacity of URM walls reduces with increasing wall size. Such a size effect is of concern since most wall tests were carried out on test specimens with heights much smaller than actual story heights. In modern URM buildings the wall height is, however, often equal to the story height. Current drift capacity models implemented in structural design codes do not account for this size effect, thereby they tend to overestimate the drift capacity of URM walls with heights equal to the story height. The objective of this paper is to review existing evidence for size effects on the drift capacity of URM walls and discuss possible reasons for this effect. The paper starts with a general review of size effects in quasi-brittle structures and a review on existing numerical and experimental evidence for size effects in the seismic response of URM walls. It puts forward the notion that for walls failing in flexure the size effect is largely caused by the confining effect of the foundation, which diminishes with increasing height, while for walls failing in shear the size effect stems mainly from the post-peak response and the formation of a crushing band of the height of a brick. It concludes with an outlook on future research needs for quantifying the size effect on the drift capacity of URM walls in flexure and shear.