Blob.size

var sizeInBytes = blob.size

// fileInput 是個 HTMLInputElement: <input type="file" multiple id="myfileinput">
var fileInput = document.getElementById("myfileinput");

// files 是個 FileList 物件 （類似 NodeList）
var files = fileInput.files;

for (var i = 0; i < files.length; i++) {
  console.log(files[i].name + " has a size of " + files[i].size + " Bytes");
}