To change the font size in HTML, use the style attribute. The style attribute specifies an inline style for an element.  The attribute is used with the HTML <p> tag, with the CSS property font-size. HTML5 do not support the <font> tag, so the CSS style is used to add font size.

Just keep in mind, the usage of style attribute overrides any style set globally. It will override any style set in the HTML <style> tag or external style sheet.