Change font size of Portfolio items.

To change font size of following portfolio items title .... 

Enter below style in Themeoptions/general/display/customcss box.

article.hentry .portfolio-item.one-three .portfolio-title h2{font-size:15px;}

Change 15px to your liking.

Note that above style applies only to 3 column portfolio, if you want to apply it to all 3 types of portfolios, use below code.

article.hentry .portfolio-title h2{font-size:15px;}