Introduction Though the WHO Framework Convention for Tobacco Control (FCTC) calls for the implementation of large graphic warning labels on cigarette boxes (GWLs), the courts have blocked the implementation of 50% labels in the U.S. We conducted an experiment to explore whether changing the size of GWLs is associated with changes in visual attention, negative affect, risk beliefs, and behavioral intentions. Method We recruited adult smokers (N = 238) and middle-school youth (N = 237) throughout New York state in May 2016. We randomly assigned participants to one of three between-subjects conditions (no GWL [control], 30% GWL, 50% GWL). Results Adult and youth participants looked at the GWLs longer when the GWL covered 50% versus 30% of the pack's front. Increasing GWL size from 30 to 50 percent did not influence negative affect or risk beliefs, though both GWL sizes increased negative affect relative to the no-GWL control group. Exposure to 50% GWLs increased adult smokers' intentions to quit compared to no-GWL, but smokers exposed to 30% GWLs did not differ from control. There were no differences between 50% GWLs, 30% GWLs, or control on youth smoking susceptibility. Conclusions Findings provide some evidence of the benefits of a 50 versus 30 percent GWL covering the front of the pack for adult smokers and at-risk youth from socioeconomically disadvantaged backgrounds-though not on all outcomes. Implications This research shows that 30% GWLs on cigarette packages increase negative affect relative to packages without front-of-package GWLs. Larger GWLs on cigarette packages (50% versus 30%) increase visual attention to the warning and its pictorial content among low-SES smokers and at-risk youth but do not further increase negative affect. A 50% GWL increased adults' quit intention compared to no GWL at all, but we were underpowered to detect modest differences in quit intentions between a 50% and 30% GWL. Future work should thus continue to explore the boundary conditions under which relatively larger GWLs influence cognitive, affective, and behavioral outcomes.