You may have zoomed in by mistake. Depending on the browser you use, a simple trick is to hold the 'Ctrl' button on your keyboard and click the '-' button to zoom out. This is located across the top row of numbers next to the '+' button.
To zoom in click 'Ctrl' and the '+' button. When doing so the percentage will change at the top of the page.
Alternatively use your browser settings to find the Zoom option. For Google Chrome, (our recommended browser) this shows in the Settings menu itself when you click the three dots in the top right-hand corner of the web page.