You can use an @media rule to target mobile devices based on screen size. Example:
@media screen and (max-width: 800px) { tw-story { font-size: 125%; } }
Harlowe 1.2.4 uses em units to define it's font & line-height sizes, it might be a good idea to use the same units.
Here is a link to information about using media queries in general, and an article (*) that explains about using breakpoints to target different view-port / screen sizes.
(*) The article is about HTML emails but the information in it is relevant for HTML web-pages as well.