To change the number of measures that are generated:

  1. Generate some music
  2. Click/tap the Gear icon on the toolbar (next to the Tempo) which will open the Settings sidebar. 
  3. In the Settings sidebar, click the dropdown in the 'Measures' section to change the number of measures (see the screenshot below)
  4. Now when you generate a new exercise, the measures will change to what you just set