To identify behavioural mechanisms and determine their influence on the mating pattern in a population of Hyla labialis, we integrated sampling results from the field with results from manipulated laboratory observations. We found no support for a large-male mating advantage based on male-male competition in shaping mating pattern. Members of naturally occurring mated pairs had body sizes representative of the entire population, and revealed a size-assortative mating pattern. This trend also appeared in the laboratory, when a gravid female had the opportunity to choose among two males of different body sizes. Larger females laid larger egg clutches, but larger males did not fertilise more eggs. However, the clutches of females clasped by proportionately larger males led to a higher percentage of hatchlings, suggesting that size-assortative mating improves fertilisation rate through cloacal apposition during spawning. Despite a significant negative correlation between male body size and dominant frequency of the advertisement calls, providing acoustic cues for females to discriminate among males, it seems that female mate choice leading to size-assortative mating is based only partially on call characteristics.