Decoded, that says “for an HTML page’s body section, the container for all the stuff you see on a page, set the font family to Arial and override everything else.” ...

Click the Photoshop document and type the text. You can also double-click a text layer in the Layers pane, which highlights the text, and then use the font-size selector to increase the font size.

Text size is affected by multiple factors including browser default styles, styles added by the HTML document, browser settings, and sometimes user style sheets.