When selecting select, mark the desired checkbox by the data value of the selected select value?

 $("#select").on("change", function(){ $("input[type='checkbox']").prop("checked", false); $("input[type='checkbox'][data-size='"+$(this).find("option:selected").data('size')+"']").prop("checked", true); }); 

 <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <select class="select" id="select"> <option value="all" data-size="all" class="subSelect">Все размеры</option> <option value="big" data-size="big" class="subSelect">Большие</option> <option value="middle" data-size="middle" selected class="subSelect">Средние</option> <option value="small" data-size="small" class="subSelect">Маленькие</option> </select> <div class='boxes'> <label class="box"> Маленький <input type="checkbox" data-size="small" class="checkbox" value="small"> </label> <br> <label class="box"> Средний <input type="checkbox" data-size="middle" class="checkbox" value="middle"> </label><br> <label class="box"> Большой <input type="checkbox" data-size="big" class="checkbox" value="big"> </label><br> </div> 

 document.getElementById('select').addEventListener('change', function() { let chekbox = document.querySelectorAll('input[type=checkbox]'); for(let i = 0; i < chekbox.length; i++) { chekbox[i].checked = (chekbox[i].dataset.size == this.options[this.selectedIndex].dataset.size) ? true : false; } }); 

 <select class="select" id="select"> <option value="all" data-size="all" class="subSelect">Все размеры</option> <option value="big" data-size="big" class="subSelect">Большие</option> <option value="middle" data-size="middle" selected class="subSelect">Средние</option> <option value="small" data-size="small" class="subSelect">Маленькие</option> </select> <div class='boxes'> <label class="box"> Маленький <input type="checkbox" data-size="small" class="checkbox" value="small"> </label> <br> <label class="box"> Средний <input type="checkbox" data-size="middle" class="checkbox" value="middle"> </label><br> <label class="box"> Большой <input type="checkbox" data-size="big" class="checkbox" value="big"> </label><br> </div>