So you’re trying to make sure that the file size is downloaded before moving onto the next steps, correct?
The link you mentioned is simply returning a Boolean (True/False) value. If you want it to wait until the path exists, you should structure your workflow so that it uses the Path Exists activity to look for the file you just downloaded. If path exists = False, then check again after a set amount of time (delay activity). If path exists = True, then continue onto the next activity.
Doing this would make sure the robot waits until each file is downloaded before continuing the process