In Affinity Photo there are options to resize the canvas of opened images and newly created documents.
Changing the canvas size involves adding or taking away pixels from around the edge of your image, much like adding a border to a picture or cropping the image to a smaller size, respectively. Unaffected pixels are not stretched or squashed.
Unwanted canvas area can be removed (trimmed) from the document edge, and conversely, you can expand your canvas to fit layer content which extends over your document edge.
To change canvas size:
From the Document menu, select Resize Canvas.
Enter your new canvas dimensions in the Size boxes—left box for width, right box for height.
To resize the width and height independently, click the lock icon (to unlock) between the Size boxes.
(Optional) Select a different measurement unit from the Units pop-up menu. Rulers will update to the new measurement unit.
In the Anchor box, select alignment options to resize the the canvas 'under' the image from that anchor point.