WaitFileComplete
This utility checks the file size every so many seconds. If the size doesn't change a specified number of times in a row, WaitFileComplete will exit, and the remainder of your .bat file will run

Example:
waitFileComplete -seconds 2 -retries 10 "%your_file%"

The above command would check the file size  of %your_file% every 2 seconds. 
If the size doesn't change 10 times in a row, WaitFileComplete will exit, and the remainder of your .bat file will run.

WaitFileComplete /?
Usage:
WaitFileComplete <options> filename
Valid options:
  -seconds 99   The number of seconds between checks
                If not specified, defaults to 10.
  -retries 99   The number of times the filesize should be
                the same before WaitFileComplete considers
                the file to be complete
                If not specified, defaults to 2.
  -verbose      Prints extra information while checking.