1e928c53_nsIChannel___Mozilla___MDN__2
[0] contentLength [1] int64_t [2] 
The length of the data associated with the channel if available. A value of -1 indicates that the content length is unknown. Note: As of Gecko 19.0, this parameter changed from long to int64_t. In earlier versions callers could get the "content-length" property as 64-bit value by QueryInterfacing the channel to nsIPropertyBag2, if that interface is exposed by the channel.
nsIChannel - Mozilla | MDN