How do I change the font size used in the caption of image blocks?

How do I change the font size used in the caption of image blocks on a page-per-page basis?

I was using this pathway: 'Manage Site' (gear icon) > 'Page Settings' > 'Advanced' > and pasting this into the 'Page Header Code Injection' section:

<style>
  .sqs-block.image-block .image-caption-wrapper p { font-size: 18px; }
</style>

This does change the font size, but when I go to the website the font size only changes if I refresh the website page and is the old font size when initially clicking on the page. Is there a solution to make this font size permanently display?