For configure your default canvas size you need to:
- Open a new document:

2. Set you preferred width and height:

3. Click in save preset

4. Give it a name to your saved settings:

5. Open again a new document, every time you open a new document you can go to Preset and slide your dropdown and select your saved settings:

6. Finally click in OK.

Here you have a video if you prefer do it in this way.