Stabilizing selection is a type of selection in which a non-extreme trait is benefited and become the population mean. So the individual that would have a bigger fitness would be the ones that possess the middle traits alongside all possible traits. Talking about body size of lizards, a stabilizing selection would benefit the average-size ones, and would impair the smaller and bigger size ones. For example in the desert a small-size lizard could have a hard time competing for resources, but as lizards grow in size become more actively prefered and hunted by predators such as birds or snakes, so an average-size lizard would have a balance between access to resources and survival, giving it an evolutionary advantage, thus stabilizing the selection in that population.