The exact rule is going to depend on whether you want the entry titles to be reduced everywhere or just on blog lists or whatever.
Lets say you want the font size for titles to be 1.8em.
Now, if you want that to be the case everywhere, then this should work:
#content h2 {
font-size: 1.8em;
}
If, however, you only want this to be the case for posts you would have to do something like:
#content .type-post h2 {
font-size: 1.8em;
}
But if you wanted the title to be larger (say 2em) for the single entry page (as opposed to the list), you would also have to add something like:
#content .type-post h2.entry-title {
font-size: 1.8em;
}
#content .type-post h2.single-entry {
font-size: 2em;
}
So you need to decide exactly which titles you want to display in a smaller font and then aim your CSS at exactly those you want.
Then you need to test all the possible situations. For example;
- blog lists
- category lists
- single page blog entries
- page display (as opposed to post)
- tag lists
- etc
As usual, I would urge you to do this in a child theme if that’s at all possible. If you do that, you just need to add the rule(s) you decide on to the end of your child theme’s style sheet.
HTH
PAE