HTML

change the font-size on a label css

.group {
  margin-bottom: 20px;
}
.group label {
  font-size: 10px;
}
.group2 label {
  font-size: 4px;
}

change the font-size on a label html

<div class="group">
  <label for="input1">First label</label>
  <input type="text" name="input1">
</div>
<div class="group group2">
  <label for="input2">Second label</label>
  <input type="text" name="input2">
</div>