Copy the attached behat_size.feature file to lib/tests/behat Initialise behat. (e.g. mdk behat ) Run the scenario: vendor /bin/behat --config= /path to moodle instance's /moodledata_behat/behatrun/behat/behat .yml --name= "Test the screen size" Every time the scenario pauses, make a note of the screen size set the WINDOWSCALE environment variable to 3 : export WINDOWSCALE=3 Run the scenario again. Every time the scenario pauses, make a note of the screen size Compare the screen sizes They should be 3 times larger