Bear in mind that it is possible for file data to change without the file size changing; for example, some data fields in the initial "root blocks" of the iso file might be filled in as a final step, after the full extent of data files have been written. So rather than checking file size, it might be better to check the modification time (using  -M $isofilename).

But as others have pointed out, the best solution is to make a slight change in the process that creates the iso file: the directory or name of the file being created should be different from the directory or name used to download the file; just add a step after the "mkisofs" or whatever, to rename/move the completed file so that the download process will only find it after it's finished.