Many studies have shown that vegetation structure and habitat complexity affect taxonomic composition, functional diversity, and the number of individuals in spider assemblages. These factors also affect spider body size, but mechanisms responsible for that are still not well understood. In our research, we examined the relationship between the body size of spiders from two species – Alopecosa cuneata and A. pulverulenta – and environmental factors such as habitat type and habitat complexity. Our research was conducted in the Bug River Valley on 12 plots covering three types of habitats: mesic meadow, sandy grassland, and xerothermic grassland. Spiders were collected in 2007-08 from April to mid-November using pitfall traps. In total we measured 334 males and 168 females of Alopecosa cuneata and 315 males and 104 females of A. pulverulenta. The generalized linear mixed model revealed that individuals of Alopecosa cuneata as well as A. pulverulenta reached larger sizes in more complex vegetation, whereas the habitat type did not affect the spider body size. One of the likely mechanisms responsible for a larger body size in more complex habitats is predator pressure. Birds as the main predators of spiders, being selective in their choice of prey, may collect larger spiders with higher intensity than smaller ones. We suggest that more complex habitats with dense vegetation provide better shelter for large spiders, which allows them to avoid predators. Our results indicate that habitat complexity may be an important determinant of body size distribution in spider assemblages.