Under the drop down menu for the Region Tools, you'll find the Frame Tool. To add a Frame to your Canvas:

  1. Select "F" (or "A" if you're more familiar with the term Artboard)
  2. Drag out a Frame onto your Canvas

Manually entering the dimensions of your Frame in the Inspector on the right side...

Or select the drop down menu next to Frame, and select from a number of predefined sizes. 

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