https://web.fredden.org/assets/file.js

1function checkSize() {
2  // check for browser support (may need to be modified)
3  if (!this.files) return;
4  if (!this.files.length == 1) return;
5
6  var size = this.files[0].fileSize || this.files[0].size;
7  var ext = "B";
8  if (size > 1024) { ext = "KB"; size = size / 1024; }
9  if (size > 1024) { ext = "MB"; size = size / 1024; }
10  if (size > 1024) { ext = "GB"; size = size / 1024; }
11
12  this.title = "File size detected as " + size.toFixed(1) + " " + ext + ".";
13
14  if (ext == "GB" || (ext == "MB" && size > 6)) { // Max 6 MB
15    alert("File uploads are limited to 6 MB, your file is " + size.toFixed(1) + " " + ext + ".");
16    this.value="";
17  }
18}
19
20window.addEventListener('load', function() {
21  var inp = document.getElementById('upload');
22  inp.addEventListener('change', checkSize);
23});