This article helped me to understand iOS screen sizes.
Farah Saud

About android, there are templates by Google in Sketch. If you go to ‘File > New From Template > Material Design’ you will find those resources. Google has used an art board of the size 360x640 px. Not to confuse you too much, use this resolution to design for android.

When you want to hand out your designs to your developer and the assets along with it, I highly recommend using Zeplin for that. That way you do not spend countless hours in creating assets for each resolution. In zeplin the developer can just select any element, export it and they will get all the possible resolutions neatly tucked into separate folders without your intervention.

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