- 45x45px for 1x devices (iPhone 3G and 3GS)
- 90x90px for 2x/Retina Display devices (iPhone 4, 4S, 5, 5S, and 6)
- 135x135px for 3x devices (iPhone 6 Plus and up)
So in what is possibly the best news I've heard last year at WWDC, Xcode 6+ now supports vector PDFs in Asset Catalogs. Your designers will know what this means but essentially, PDFs are the de facto standard for vector assets. A vector file contains a lot of metadata of an asset that tells the system how to render it's contents, independent of the screen's resolution. Boiled down as simply as possible, an image of a circle in a vector PDF will be just as crisp and clear when rendered at 5 pixels wide as it will be when rendered at 5,000,000 pixels wide without seeing strange artifacts or pixelation.
How Xcode does this is by generating your 1x, 2x, and 3x images at compile time based off of the size of your vector PDF's assets for iOS platforms. If your vector PDF image is 45x45px, then Xcode will generate three PNGs at compile time:
This also means that whenever we get larger screen resolutions, Xcode will be able to scale up your images from your already existing vector PDF for you, giving you automatic support for future devices for free. Aaaannndd, if you happen to be an OS X developer, then it gets even better for you! Vector PDFs have full support in OS X apps, giving you complete ability to scale up your images in code without distorting it.