Came across open-iconic for a react issue recently and saw this question un-answered; for normal (non reactJS) use of open iconic, the font-size property would work as shown below;
working snippet with bootstrap v3:
span {
font-size: 30px;
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.0/css/bootstrap.min.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/open-iconic/1.1.1/font/css/open-iconic-bootstrap.min.css" integrity="sha256-BJ/G+e+y7bQdrYkS2RBTyNfBHpA9IuGaPmf9htub5MQ=" crossorigin="anonymous" />
<span class="oi oi-sort-ascending"> </span>
<span class="oi oi-person"> </span>
working snippet with bootstrap v4:
span {
font-size: 30px;
}
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-beta/css/bootstrap.min.css" integrity="sha256-m/h/cUDAhf6/iBRixTbuc8+Rg2cIETQtPcH9D3p2Kg0=" crossorigin="anonymous" />
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/open-iconic/1.1.1/font/css/open-iconic-bootstrap.min.css" integrity="sha256-BJ/G+e+y7bQdrYkS2RBTyNfBHpA9IuGaPmf9htub5MQ=" crossorigin="anonymous" />
<span class="oi oi-sort-ascending"> </span>
<span class="oi oi-person"> </span>