In Affinity Designer you can resize your document or canvas. The former lets you scale on-page objects in proportion with the new document size, while the latter lets you grow or shrink just the canvas with a choice of how to anchor the canvas on resize (objects will never resize).
Changing the document size will increase/decrease object dimensions in relation to a change in document Width and/or Height. In doing so you have a choice of resampling methods to control the quality of scaled objects.
For canvas resizing, the canvas is independent of the page and can be resized in relation to a configurable anchor point (e.g., Middle Center). As objects don't scale, resampling is not required. This is useful for expanded your working area if you're envisaging a bigger design than initially thought.