ah thats frustrating!! no idea to be honest, sorry, canvases should be scaling :(. At the very bottom of the screen there might be a small blue icon (on itch) to force it into fullscreen, aside from that i have no idea, ill look into it. Is there a specific downloadable build i could post that would help?
*update* - forcing it to run at 2560 x 1600, im not seeing the same issue. Must be something web specific, in which case im not sure how to fix it!! Again id be happy to try and supply a build for you :)