File size does matter.
It gives you a chance to hit buttons that are supposedly inactive but are actually active.
One should wait till an image of the file appears in the big box before clicking the Add button.
That's supposed to give you a chance to view the file a bit and be able to kill the upload if it really isn't the one you want.
I can pretty well guarantee that lots of systems puke when you start pushing controls in the middle of operations.
Maybe they shouldn't but patience is a virtue.
Systems typically give you some kind of indication when a very lengthy operation is completed successfully. Hitting buttons prior to that is asking for trouble