Lizyd Design is correct.
You can use
h2 {
font-size: 10px;
}
and when you will use h2 tag it's font size would be 10px
or
you can try below
.myfontClass {
font-size: 10px;
}
and call this class in h2 tag like below
<h2 class="myfontClass">
This will help you change font size in h2 where...