Zooplankton body size is a trait that co-varies with a spectrum of different fitness-related variables, including swimming velocity, feeding rate, metabolic rate, visual conspicuousness, and susceptibility to different guilds of predators. Here we illustrate a fundamental trade-off associated with body size, since both predation risk - due to visual predators - and predator avoidance behavior are a positive function of body size. There is not a single optimum balance between size-related risk and predator avoidance, expressed as diel vertical migration (DVM) behavior. Instead, this balance changes with body size in a nonlinear manner. Empirical support for a size-dependent trade-off comes from the California Current Ecosystem LTER site, which includes waters of widely disparate optical characteristics. Encounter rates with visually-hunting predators are dependent upon zooplankton body size and are therefore sensitive to variations in optical transparency. The ability to avoid predators, through diel vertical migration (DVM) behavior, is dependent upon body size-related swimming ability. Our results show that the amplitude of DVM behavior varies widely with copepod body size: smaller-bodied copepods in this region tend to show low amplitude DVM, remaining in near-surface waters day and night; intermediate body-sized copepods exhibit DVM behavior; and the very largest-bodied copepods also show little evidence of DVM, but remain in deeper waters. We found that the amplitude of copepod DVM is directly proportional to the optical transparency of the water column. Therefore, the long-term increases in optical transparency that we have previously documented in the CCE region are expected to shift the vertical distributions of planktonic copepods, which are key members of pelagic food webs.