Font sizes on Mobile

Hello,

I’d like to modify the font size of the site title to make it a little larger on mobiles and tablets (when width is lower than 630px).

I’ve tried a custom css using the following code but it didn’t work.
@media all and (max-width: 630px) {
#site-title {font-size: }
}

I’m also trying to reduce the size of the post titles.

Any suggestion?

Thank you.