reduce size of h1 and h3 tag?

If you’re referring to the size of the font, you can change the size of those by putting a CSS rule into your custom CSS folder.

h1 { 
    font-size: 12px;
}

Insert the size you want your font to be in the font-size field. Repeat for H3 tags.

h3 { 
    font-size: 12px;
}

You can also use CSS to change other parts of the H1 and H3 element.

If you do not have a custom CSS file you will find the rules for your H1 and H3 elements in style.css in your WordPress install.

Your H1 may be displayed in your CSS file as

h1 {attributes}

or

#header h1 {attributes} 

Hope that helps!