I was looking at the common CSS in the GitHub link. Having a bit of trouble identifying element.
But if you can identify the element you should be able to create a custom component and use something like
font-size: 44px !important;
To override the H1 size in the banner.
If you have better CSS knowledge then myself you might have an easier time identifying the element. Try inspect element as I am using a mobile with the insoect element in a browser but it is not as good as using a desktop browser’s insoect element. You can test modifying size there.
Posting In the advance Search Banner topic asking how to change the H1 size you might also get some guidance on how to achieve it as well.