Positive correlation of species richness with area is ubiquitous in nature, but the processes driving that relationship, and those constraining typical patterns, remain elusive. Patch size variation is pervasive in natural systems, and thus it is critical to understand how variation in patch size, as well as its potential interaction with factors like predation and isolation, affect community assembly. We crossed patch quality (fish presence/absence) with patch size to examine effects of quality, size, and their interaction on colonization by aquatic insects. Overall, beetles favored small, fishless patches, but individual species sorted across patch size, while hemipterans aggregated into large, fishless patches, producing sorting between Coleoptera and Hemiptera. Both size and predation risk generated significant variation in community structure and diversity. Patch size preferences for the 14 most abundant species, and pre-eminence of species turnover in patterns of beta-diversity, reinforce patch size as a driver of regional species sorting via habitat selection. Species sorting at the immigration stage plays a critical role in community assembly. Identifying patch size as a component of perceived quality establishes patch size as a critical niche dimension, and alters our view of its role in assembly dynamics, and the maintenance of local and regional diversity.