you can create a White list entry for the Type "Header" -> "Content-Lenght greater than" and specify a size. Disable all filters for this entry.
This will work for all downloads which sent the Content-Length Header. If a server does not sent one (should be uncommon) and uses chunk-encoding for sending the file, we need to have the complete file first before we know how big it is. No chance to bypass here.
This should not happen too often. You would notice it when downloading without webwasher that the browser can't tell you the size and remaining time. Did not come across this pretty often in the last time.
Thanks Andre, I was not aware about this option.