I could not find a simple setting to change the font size of the post titles, but I could find a simple way to do that by editing the html code.
1. Go to the home page of your Tumblr account and click on 'Customize' menu
2. In the left side panel, select 'Edit HTML'
3. Search for 'block:Posts' section in the html code. (use ctrl + f to get the search box )
Then you can see the 'block:Title' section in that. You can add the font style that you need for the title of the posts in that section.
4. You can use 'Update Preview' button and see how the new font style looks like.
Make the changes you need and hit 'Save' and you have what you need.