It seems that the portfolio filter font is linked to the body font style. Is there any way to specify a different font and size for the portfolio filters apart from the description font used in the single post layouts?
Yes, you can do this with custom css. Add this code into your custom css box:
#portfolio-filter ul li a {
text-transform: uppercase;
letter-spacing: 1px;
font-weight: 600;
font-family: Great Vibes;
color: #bebebe;
font-size: 22px;
}