A utility loader incorporating a boom of adjustable length. The boom may include either or both of a left and right lift arm assembly, with each lift arm assembly including a rear lift arm that telescopically receives a front lift arm. Each lift arm assembly also includes an extension actuator adapted to telescopically extend and retract its front lift arm relative to its rear lift arm. A detection system may be provided and adapted to limit lift arm assembly extension based at least in part upon a load applied at a tool supported by the boom and a relative telescopic location of a front lift arm relative to its associated rear lift arm.