Adjusting canvas size for online experiments

Description of the problem:
The canvas size (the size of the visual surface in the browser / the size of the page) in my experiment seems to correspond to the full-screen size.
This works great in the full-screen mode. However when I am running the task out of the full-screen mode, this canvas is larger (in both height and width than the browser window), and the browser shows scroll bars.

This is highly undesirable because left and right arrow keys, which participants use to respond, end up shifting the canvas and all the task objects left and right every time they respond.

Is there any way to avoid this? E.g. rather than fixing the canvas size it to 100% of the screen size, taking 90% instead, or setting it to whatever size the browser window is?

URL of experiment: TBDTask [PsychoPy]