Reducing title font size on mobile phone

As suggested in the FAQ section I used the following code to reduce title text on mobile devices

@media only screen and (max-width: 320px) {
h1.hestia-title{
font-size: 16px !important;
}
}

This works fine on my iphone5 but on my partner’s Samsung which has a wider screen this does not work and the text is still way too big. I’m thinking I may need to edit that max-width number for the screen?? But I’m scared of breaking things!

Any advice much appreciated

Thanks
Andrew