How to change the font size of the image box

Hi,

I havent been able to change the font -size of the titles of specific image boxes which are h3, I know it has to be done by adding a name in the extra class name and then refering to it in the css, but I haven’t figured it out. (I dont what to change the font size of all the h3 images boxes I just need to change some titles of some specific image boxes)

Do you know how can I do that?

Thanks in advance,

David