At this time, you are not able to change the canvas size in Sketch. Sorry about that! Please share your feedback through the "Give us Feedback" link within the main menu of the app (tap the product icon in the top left corner > "Give us feedback").
Let me know if you need anything else!
I'm pretty sure that currently, the available canvas size of Adobe Sketch is 2048 x 1536 (at least).
Fortunately, HD Video is smaller (1920 x 1080).
Create a 1920x1080 template in Photoshop, then save it to your CreativeCloud library. It's often helpful to create a guide with title safe and action safe too!
In Adobe Sketch, place the template into a new layer and use it as your guide layer. If you send this to Photoshop later, it'll be really easy to crop to that guide layer - and maintain your elements and design layout.