JSFiddle - React, Tailwind, and code Playground

HTML

<input type="file" id="file-multiple" multiple>
<p>Reported file size - multiple: <span id="result-multiple"></span></p>
<input type="file" id="file">
<p>Reported file size - single: <span id="result"></span></p>

JavaScript

document.getElementById('file-multiple').addEventListener('change', function (event) {
    var fileSize = event.target.files[0].size;
    document.getElementById('result-multiple').innerHTML = fileSize;
}, false);
document.getElementById('file').addEventListener('change', function (event) {
    var fileSize = event.target.files[0].size;
    document.getElementById('result').innerHTML = fileSize;
}, false);