You should use the following canvas sizes for different iOS devices, when you need to specify the exact aspect ratio:
Any iPad: 768 x 1024
iPhone 4: 320 x 480
iPhone 5/5s: 320 x 568
iPhone 6/7/8: 375 x 667
iPhone 6/7/8 Plus: 414 x 736
iPhone X: 375 x 812
You can use the 50% scale option for buttons and images where appropriate to use the full capability of a retina device.
These options are selected automatically if you choose the appropriate device when creating a new project. Note the AutoSize function will scale a project to whichever device you load it to