Drop an ad in. Width is locked to the mobile feed. Then switch on real phone size to see it at true physical dimensions, the size it actually is in someone's hand. If you have to squint here, the audience can't read it either.
Hold a real credit or ID card (any standard bank card, 85.6 mm wide) against the screen and drag until the bar matches its width exactly. This makes "real phone size" mode show the ad at true physical inches.