Setting up the edit-time Canvas

Within the Viewport pane, you can find the Canvas, a floating re-sizable edit-time container that contains a live version of the UI Document (UXML) being edited. If you can't see it, try clicking on the Fit Canvas button in the Viewport toolbar to bring it into view. Any settings related to the Canvas, like its size, are not saved as part of the UI Document (UXML) but will be remembered (using an internal separate settings file) for the next time you open the same UI Document (UXML).

You can directly resize the Canvas inside the Viewport by dragging its edges or corners. For exact sizing, you can click on the header of the Canvas to access its settings via the Inspector pane, where you will see fields for Canvas height and width. In the same section, you can also lock the Canvas size to the size of the Unity Game Window using the Match Game View checkbox to better match a runtime UI environment:

With the Canvas selected, in the Inspector, you can change the Canvas background to make editing the UI in context easier. You can set it to be a solid color, a specific texture (ie. a mockup from a UI Designer), or a live view from a Camera in the currently open Unity Scene: