In the backend of your joomla site, go to

extensions/template manger --> tab: Templates --> click template you wanna edit,
Here you'll find all the css files, amongst which is the override.css (don't forget to turn the "override settings" ON in your template manager / Edit style / under Advanced Settings!

click Edit css/override.css

enter the text Kannan suggested (which works fine, thank you, Kannan)


div.itemHeader h2.itemTitle {
font-size: 36px; // just change the value
}

and save and close the file.
e.g.: I changed the value just fine into 20px.

Regards,
Peter