Abstract: Jumping spiders (Salticidae) are known both for their excellent vision and for the elaborate visual displays they use to mediate intraspecific interactions. Despite appearing well adapted for visual exchange of information, several correlative studies have suggested that jumping spiders, including Servaea incana, may not visually assess opponent size during agonistic interactions. To more directly examine whether jumping spiders are able to visually assess opponent size, we examined how males of S. incana respond to videos of agonistic displays from various sized conspecifics. Videos of small and large spiders were manipulated so that each opponent exemplar was presented at four sizes that spanned the range of natural variation. Tendency of live spiders to display at and approach videos were related to both their own size and the final size of the video opponents, but not the original size of the video opponents. Small spiders were less likely to display towards large opponents than small opponents whereas large spiders were more likely to display towards large opponents than small opponents. S. incana jumping spiders appear to possess the sensory ability to visually assess opponent size, but their slow visual system and the dynamic behaviour of the opponents during contests may reduce opportunities for visual assessment to the point where individual size-associated tendencies come to dominate decisions of persistence, escalation and retreat. Significance statement: To limit the costs of fighting, animals may assess their opponent’s fighting ability to decide when to withdraw from contests. Alternatively, animals may withdraw from contests when they reach a cost threshold, regardless of the ability of their opponent. Studies with live interactions suggest that jumping spiders rely on the latter strategy, despite their use of ritualised displays and excellent vision. Using video playback techniques, we demonstrate that Servaea incana males are able to visually assess their opponents during contests. The probability of displaying towards and approaching a video opponent depended upon both a spider’s own size and the size of the video opponent. However, an interaction between these factors suggests that there may be limited opportunity for assessment in live interactions and that the costs and benefits of opponent assessment may depend on an animal’s own fighting ability.