<!DOCTYPE html>
<style>
div {
  width: 10ch;
  background-color: green;
  color: green;
  font-family: Monospace;
  word-break: break-word;
  margin-top: 1px;
}
</style>
<div style="font-size: 10px;">should fit</div>
<div style="font-size: 11px;">should fit</div>
<div style="font-size: 12px;">should fit</div>
<div style="font-size: 13px;">should fit</div>
<div style="font-size: 14px;">should fit</div>
<div style="font-size: 15px;">should fit</div>