<form enctype="multipart/form-data" name="form1">
      <input type="file" name="file">
      <input type="button" value="取得图片的大小" onClick="getSize()">
    </form>
    <img name="pic" alt="在这儿显示图片">

function getSize(){
        size = 0;
        imgURL = form1.file.value;
        imgURLsplice = imgURL.split("\\").pop();
        imgURLsplicelocal ="../assets/img/" + imgURLsplice;
        img = new Image();
        img.src = imgURLsplicelocal;
        pic.src = imgURLsplicelocal;
        if (size <= 0){
            size = img.fileSize / 1024;
        }
        alert("图片大小为：" + size + "KB");
    }