cy.viewport()

To set the viewport size and dimension, use the cy.viewport() command.

cy
  .get('#navbar').should('be.visible')

cy.viewport(320, 480)

// the navbar should have collapse since our screen is smaller
cy
  .get('#navbar').should('not.be.visible')
  .get('.navbar-toggle').should('be.visible').click()
  .get('.nav').find('a').should('be.visible')

// lets see what our app looks like on a super large screen
cy.viewport(2999, 2999)

// We added a cy.wait() between each viewport change so you can see
// the change otherwise it's a little too fast to see :)
cy
  .viewport('macbook-15')
  .wait(200)
  .viewport('macbook-13')
  .wait(200)
  .viewport('macbook-11')
  .wait(200)
  .viewport('ipad-2')
  .wait(200)
  .viewport('ipad-mini')
  .wait(200)
  .viewport('iphone-6+')
  .wait(200)
  .viewport('iphone-6')
  .wait(200)
  .viewport('iphone-5')
  .wait(200)
  .viewport('iphone-4')
  .wait(200)
  .viewport('iphone-3')
  .wait(200)

cy
  .viewport('ipad-2', 'portrait')
  .wait(200)
  .viewport('iphone-4', 'landscape')
  .wait(200)