You can change the font-size of parts of your message in the Quick Announcement Bar by adding style tags (HTML code) in the message. Simply place any text you want resized inside <span> tags with styling code.
For example, this resizes part of the message to a font-size of 24px:
In the Message field:
“First part of message <span style="font-size: 24px;">Large Text!!</span> and back to small text“
The font size in the first and last parts of the message, as well as the button, is set in the Style Configuration → Font Size