Males of the autumn spider, Metellina segmentata (***Araneac: Metidae), compete for access to mates by guarding the orb webs of mature females. We investigated the influences of relative male righting ability and resource value on fighting behaviour by staging interactions in the field on webs occupied by females. In these contests, the larger male nearly always defeated its opponent when it was at least 10% greater in size. For smaller size asymmetries between opponents, the male previously resident on the female's web usually won the contest. Contest duration decreased exponentially with increasing size asymmetry between opponents, as predicted if each male assessed its relative size and adjusted its fighting strategy according to its likelihood of winning. Contest duration was also greater when the prior resident was the lighter opponent, or when size and residency asymmetries favoured different opponents as winners. Prior residents fought longer over larger, more fecund females, indicating an adjustment of fighting effort according to assessments of resource value. In contrast, intruders did not increase their fighting effort over larger females, suggesting an inability to assess female size quickly and accurately. Assessment appears to reduce the costs of settling conflicts, but imperfect information can result in inaccurate assessments and unexpected outcomes. Assessment strategies are used by other types of spiders to resolve contests, but this appears to be the first evidence for such strategies among orb-web-building spiders.