Вот способ изменить размер шрифта псевдоэлемента.
.play::before{
font-family: 'Material Icons';
content: 'play_circle_filled';
position: relative;
text-transform: lowercase;
font-size: inherit;
top:1px;
left:-4px;
}
Не знаю, работает ли он для вас.Это просто попытка.Спасибо.
.play{
border: 1px solid black;
border-radius: 20px;
padding: 5px 25px;
text-transform: uppercase;
margin: 10px;
background-color: transparent;
color: black;
}
.play::before{
font-family: 'Material Icons';
content: 'play_circle_filled';
position: relative;
text-transform: lowercase;
font-size: inherit;
top:1px;
left:-4px;
}
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width">
<title>JS Bin</title>
</head>
<body>
<link href="https://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">
<div class="container d-flex flex-row justify-content-center buttons" style="font-size: 12px;">
<button type="button" class="play"><span class="playicon"></span>Play into video</button>
</div>
</body>
</html>