// set this to the ID of the meal size
var mealSize = "";

// get a node list of all the ingredients on the page
var ingredients = document.querySelectorAll('.wppizza_ingr_option.wppizza_option');

// and then loop through each one, setting the meal size
ingredients.forEach(function(ingredient){
    let dropdown = ingredient.querySelector('select.ingredients');
    dropdown.value = mealSize;
})