If the whole exercise is not fitting on the screen, please scroll down and click on the full screen button in the bottom right-hand corner of the exercise. The full screen button looks like a rectangle with gaps in the sides. Once pressed it will expand the exercise to fill the screen and make best use of the screen's size.
If the full screen button is not visible then the problem is that the browser's zoom is set higher than 100% and is cutting off part of the exercise frame. To manually adjust the zoom in your browser, use the Ctrl key and “+” or “-” combos to increase or decrease the page magnification.