Is there a way to estimate file size from www class downloaded from Dropbox

I have some issue with the www class the "www.progress" For iOS and Android. I have used www to download file from the url from Dropbox. The www.progress worked flawlessly in stand-alone mode, but for iOS and Android www.progress returns random numbers which are incorrect. Or is there any way to achieve this goal for estimate the file size? I have read all topics around and it's likely to be the "Headers"? We need to read the file size of that one, I presume. Can anyone explain about that? **Noted**: www.progress usually returns to 0.5f to 1.0f then looping until it finished loading. It's always more than 0.5f and never start with 0f Thanks!