To know the expected size, when you get the roUrlEvent, you can call GetResponseHeaders() and get the value for the content-length header.
Unfortunately, we don't currently expose a call to get the file stat info... You will need to read the file into an array to find the actual size.
ba=CreateObject("roByteArray")
ba.ReadFile("tmp:/ByteArrayTestFile")
numBytes = ba.Count()
--Kevin