Change Font Size Tittles

There are many tools available for this kind of CSS work, for example:
– Firefox with Firebug add-on: Help info on Firebug https://getfirebug.com/faq/
– Newer IE, Chrome, etc., have similar tools
– Alternately, try a dedicated CSS resource such as https://www.css-discuss.org/ – Also, consider creating a child theme otherwise all your customization will be overwritten when you next update your theme.

Alternately, use:
– If your theme has a custom CSS option, use that to add the CSS.
– If not, install a plugin, for example Simple Custom CSS; Custom CSS Manager plugin…
– Or, if you have Jetpack installed, enable its Custom CSS module.
– WordPress 4.7+ Customizer also includes custom CSS editor that enables users to make CSS changes to a theme without having to create a child theme.

Well i can add css into custome, but dont know what ??

The best advice I can give is to try to use Firebug for Firefox browser as explained above.

Go to the Appearance/custom css setting in WordPress Admin. Right click on the part you wish to change (the title) and choose “Inspect element with Firebug”. You will see the css code in your lower right corner. Copy that code into your Additional CSS field, and your change should show in the preview screen when your code is correct.

.archive_title {
color: #ccc;
font-size: 15px;
}

Here is the code for your category title on your page IBS. Change the values to what you want and look on the preview screen to see your changes…here is an example screen shot of Firebug in action. Remember to right-click on the element you want to examine…

Hope it helps you. Regards, Ed Sanders