I have a sketch prototyp file and want to export all icons with different size for iOS/Xcode. How should I do? I only know how to export an icon/image with different size, but no idea what about for all icons or images. Maybe Sketch does not support this function?
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Selecting all the icons you want to export and then make them exportable (right panel) can do the trick.
Here's a gif of me exporting three icons to three different PNG files.
(sorry for the gif quality, a higher version can be viewed here (webm) : https://giant.gfycat.com/PeacefulShyJackal.webm
After a second look at your question, I realize I did not answer at all since you wanted different sizes for your files.
To export your icons with different sizes, you can add a new export option thanks to the "+" icon (see image).
You can then export all your icons with your export choices, here's the final result in my finder: