The best way to do this is with custom CSS in your Theme Options’s custom CSS tab.
There is no standard size, so you will have to play around with numbers and find what works for you. Here’s sample code from a recent site of mine, where I also adjusted the margins on all my headings:
h1,
.h1 {
font-size: 2.441rem !important;
margin: 1rem 0 1rem;
}
h2,
.h2 {
font-size: 1.953rem;
margin: 1rem 0 1rem;
}
h3,
.h3 {
font-size: 1.563rem;
margin: 1rem 0 1rem;
}
h4,
.h4 {
font-size: 1.25rem;
margin: 1rem 0 1rem;
}
h5,
.h5 {
font-size: 1rem;
margin: 1rem 0 1rem;
}
h6,
.h6 {
font-size: 0.85rem;
margin: 1rem 0 1rem;
}
Using rem as the unit of measure makes these sizes scale as per the body font size, which you set up in the Typography section in Theme Options.