<div class="container small"> 
<b>水</b>をもっと飲もう</div>

<div class="container medium"> 
<b>水</b>をもっと飲もう</div>

<div class="container large"> 
<b>水</b>をもっと飲もう</div>

.container {
  border: solid 3px navy;
  max-width: 8ic; /* ic length unit out in chrome 106*/
  margin-bottom: 25px;
  margin-left: 15px;
  
}

.small{
  
 margin-top:25px;
 font-size: 20px;
}

.medium{
  font-size: 42px;
}

.large{
  font-size: 76px;
}