Each thumbnail on the portfolio grid is assigned a class according to its width (“content-small”, “content-medium”, “content-large”, and so on). The project title’s font size will be different for each grid block, depending on which class it’s been assigned. If you need control over the font sizes for each thumbnail size, edit the following CSS snippet, which shows the default values, and paste it into the Add Custom CSS box, on the Advanced options section of your customization sidebar:

#portfolio-grid .content-huge .project-title   { font-size: 72px; }
#portfolio-grid .content-large .project-title  { font-size: 60px; }
#portfolio-grid .content-medium .project-title { font-size: 42px; }
#portfolio-grid .content-small .project-title  { font-size: 24px; }
#portfolio-grid .content-tiny .project-title   { font-size: 18px; }

If you’d rather use the same font size for all thumbnail titles, use the following instead:

#portfolio-grid .project-title { font-size: 24px !important; }

If you need to change the font size for the categories displayed at the bottom, edit and add the following snippet as well:

#portfolio-grid .content-huge .categories   { font-size: 36px; }
#portfolio-grid .content-large .categories  { font-size: 36px; }
#portfolio-grid .content-medium .categories { font-size: 30px; }
#portfolio-grid .content-small .categories  { font-size: 24px; }
#portfolio-grid .content-tiny .categories   { font-size: 18px; }

Or to change the size for all blocks, regardless of their size:

#portfolio-grid .categories { font-size: 18px !important; }