CSS help - changing font size in headings


I'm using the mins theme and I'm having trouble with one of my page headings, where the word is too long for the mobile version and ends up being stuck over 2 lines, regardless of which font I use (although some better than others). I want to reduce the font size of the headings across the site, but have never used CSS before. Does anyone have a bit of code that will work, or can point me in the right direction?

Thanks 🙂