The length of the antenna should be roughly a quarter of the wavelength of the operating frequency. The Z-wave network is operating 850 - 950 MHz, the wavelength is approximately 33 centimetres, so an antenna length of around 8 centimetres is ideal. So shortening or extending the antena will not have impact on the range of the device.
In addition to antenna size, other factors can also affect the range of a Z-wave network, such as the presence of obstacles in the the surrounding environment and the topology of the network. Therefore, it's important to consider all these factors when designing and setting up a Z-wave network to ensure optimal range and performance.