The width and height attributes to <canvas> control the size of its backing framebuffer. So they’re not the same thing as CSS width and height. Set the “Viewport dimensions” in Itch’s controls to something large, 640x640 is good there, and the <canvas> inside will be large, but its framebuffer can still be small – the browser will scale up for you.
I don’t know how to control the canvas size in Godot, but someone in the Discord could help for sure; lots of people using Godot here.

