When you press the + button to create a new sketch, the canvas size is based directly on the size of your iOS device's screen. They are:
 

• iPad Pro (9.7-inch) - 2048 x 1536
• iPad Pro (10.5-inch) - 2224 x 1668
• iPad Pro (12.9-inch) - 2732 x 2048

• iPad Pro 12.9-inch (2nd generation) - 2732 x 2048
   

• iPad Air, iPad Air 2 and iPad (5th generation) - 2048 x 1536
   
• iPad mini 2, iPad mini 3 and iPad mini 4 - 2048 x 1536
   

• iPhone 6, 7 & 8: 750 x 1334 pixels

• iPhone 6 Plus & 6s Plus: 1242 x 2208

• iPhone X & XS: 1125 x 2436 pixels

• iPhone XS Max: 1242 x 2688 pixels

• iPhone XR: 828 x1792 pixels
  
   

If you wish to create canvas sizes not in direct relation to the device, you can tap and hold the + button to access custom canvas sizes including:
 

• 16 x 9 aspect ratio
   
• 3 x 2 aspect ratio
   
• 4 x 3 aspect ratio
   
• Square
  
   

The specific sizes of these ratios vary depending on your device and are optimized for processor speeds and the amount of RAM each device supports.

You can also change the existing aspect ratio of a canvas by tapping the More Actions icon (...) within a sketch > Edit Canvas Size and then select from a variety of presets or choose Freeform and adjust the canvas size manually.