In your WordPress dashboard go to Appearance on the right hand menu, and then to Editor which should open to edit your style.css.  Then open your web browsers Find feature.  If you don’t know how to access it then press the Ctrl + F keys at the same time.  When the search box opens type in .hentry h1.  You should see something like this:

.hentry h1 {
font-size: 18px;

At that point change the number next to font-size and px.  Don’t touch anything else or you could mess up your blog.  After you change the number press the Update File button at the bottom of the page.  Then check your blog to see how the new font size looks.  You might have to refresh your browser to see the changes take effect.  If you don’t like the size then go back to repeat the above steps until things look right.