I use an iPhone 6 trying to mirror an iPhone 6 template from the Sketch template gallery and it doesnt mirror correctly on my device. I found that if you scale your art board to 99% and then double tap your mirror app you can make it fit, but then its all pixelated and your sketch values are all 1% smaller. Very frustrating because this app has a lot of potential to quickly check if a design feels right in your hand.