What is monitor size? Do I always need to set the monitor size before generating Responsive screenshots?

In order to display the device at full scale, we need to obtain the PPI of your viewing device. We can do this by using the diagonal size of your screen. Unfortunately, there's no way for us to determine your screen diagonal size, and hence you can select and set the size yourself. Once the monitor size has been set, it is stored in a cookie and you will not have to set it again. While it is not necessary to set the monitor size, it is required in order to correctly view the device at actual scale.