Android Question Canvas size not containing my variant

The Layout Designer is built to make it easy to design layouts that will automatically fit different screen sizes. What you call the "canvas" is a notional screen area. One normally designs a layout in which the views are "anchored" to the screen edges; their sizes are specified in dips (device independent pixels). When the layout opens on a real device it automatically scales itself to fit the exact screen limits irrespective of the actual pixel dimensions. This is what most people want. Although it might appear counterintuitive you should try to design your layout in the same way if at all possible, otherwise you will to some extent find yourself fighting with the Layout Designer's expectations.

If you want to take the path of designing a bespoke layout for a single device then you can do so, of course, and set up a specific variant. But be aware of the difference between dips - the natural units used by the designer - and actual device pixels.