Download file with callback

The command CopyFile() can download a file from the internet and copy it to the user's hard drive. I want to write a new copyfile function with a callback, so that the download progress can be displayed:

Import leadwerks.lwmath

Function CopyFile2:Int(src$,dst$="",callback:Byte Ptr=Null)
	src$=Replace(src,"","/")
	src=Replace(src,"://","::")
	If dst="" dst=AppDir$+"/"+StripDir(src)
	
	If callback=Null
		Return CopyFile(src,dst)
	Else
	
		Local DownloadProgressCallback(p#)
		DownloadProgressCallback=callback
	
		out:TStream=WriteFile(dst)
		If Not out Return
		in:TStream=ReadFile(src)
		If Not in
			out.close()
			Return
		EndIf
		
		size=in.size()
		While Not Eof(in)
			WriteByte out,ReadByte(in)

			newprogress#=Float(in.pos())/Float(in.size())
			'If Round(newprogress*100.0)>Round(progress#*100.0)
			'	progress=newprogress
			DownloadProgressCallback newprogress
			'EndIf
		Wend
		
		in.close()
		out.close()
		Return True
		
	EndIf
EndFunction

win:TGadget=CreateWindow("",200,200,400,300,Null,WINDOW_TITLEBAR)
Global progbar:TGadget=CreateProgBar(10,10,win.clientwidth()-20,28,win)
CopyFile2("http://www.leadwerks.com/index.php","",DLCallBack)

Function DLCallBack(progress:Float)
	UpdateProgBar progbar,progress
	If PeekEvent() WaitEvent()
EndFunction

The problem is that a URL opened with ReadStream() has an unknown size. If I pre-cache the file with ReadFile(), the entire file is loaded, so I know the size, but I can't display the progress until after the actual file data is already downloaded.

How can I find out the size of a file on the internet, or a stream opened with ReadStream()?