JSFiddle - React, Tailwind, and code Playground

HTML

<p>
      Most people pick their board size by prefrence but I will make a tool below to choose a board size that will fit your shoe size best. The most popular board sizes are 7.75, 8, 8.25, 8.38, and 8.5. <br> <br>
      If your shoe size is: <input id='shoeSize' type="text" class="shoe" onchange="shoeSizeChanged()">The best board size for you would be: <span id="result"></span>
</p>

JavaScript

function shoeSizeChanged() {
			const shoeSize = parseInt(document.getElementById('shoeSize').value)
      
      let boardSize = ''
      switch (shoeSize) {
        case 0:
        case 1:
        	boardSize = 5;
        break;
        case 3:
          boardSize = 4;
        break;
        default:
          boardSize = '?';
		}
        
        
    document.getElementById('result').innerHTML = boardSize;
  }