If you are gonna use more icons with exact same appearance then in the downloaded css change the font-size to inherit and create a class in your local css with the required font-size, you can apply this to any property.
.fi-yen:before,....,.fi-zoom-in:before,.fi-zoom-out:before
{
font-family: "foundation-icons";
font-style: normal;
font-weight: normal;
font-variant: normal;
text-transform: none;
line-height: 1;
-webkit-font-smoothing: antialiased;
display: inline-block;
text-decoration: inherit;
font-size:inherit; #add this
}
and in your local css file
.icons{
font-size:25px;
}
now you are good to use this class in addition to your icon class. so your html will look something like this
<span class="icons"><i class="fi-social-pinterest"></i></span>
<span class="icons"><i class="fi-social-twitter"></i></span>