The design of marine protected areas (MPAs) generates much discussion among marine scientists. Ecological studies have suggested that protecting 10 to 40% of regional ecosystems is needed to preserve diversity, while public preferences for MPA size have not been modeled. We conduct a choice experiment to estimate the existence value of protecting species and habitat diversity on the sea floor in areas that vary in size and allowable uses of the water column, and apply a latent class specification to accommodate taste parameter heterogeneity. Results identify three latent classes in the sample, and suggest that while protecting areas as ecological reserves is utility-increasing for most size/use combinations, smaller reserves with liberal use policies produce the largest increases. Our research suggests diminishing marginal utility for MPA sizes that are substantially smaller than ranges often cited, and the distinct latent classes underscore the need to question the assumption of homogeneous preferences when evaluating MPA policy.