In the discussion “Adjusting the size and alignment of text in my post,” Mundobizarro asked how to increase the size of certain text in a post. pfaffman suggested using BBCode [size=xxx] tags by referencing the Discourse BBCode documentation.
Alternatively, jomaxro recommended using HTML <big>big</big> and <small>small</small> tags. awesomerobot mentioned that Discourse uses Markdown and provided a tutorial link, noting that headings can be created using # Heading 1, ## Heading 2, and ### Heading 3.
Mundobizarro found the # method helpful but then asked about center-aligning text or headings, which currently align left by default.
Without any plugins you can also use HTML big and small to resize text. Without any plugins you can also use HTML <big>big</big> and <small>small</small> to resize text.