Small lizards have difficulty defending their territory, but large lizards are more likely to be preyed upon by owls. which type of selection would act on body size in these lizards under these condition

Stabilizing selection is at work in this case. This is the force that allows a population to gradually center on a mean value due to a presence of a non-extreme bodily trait. The more extreme examples of the trait lead to the organisms being selected out, and the trait that is the least conspicuous wins out in the long-run.