Ok, here is an idea (that I have not test, leave that to you)
On global.asax
use one of this function (find the one that have a valid final size)
Application_PostRequestHandlerExecute
Application_ReleaseRequestState
and get the size that you have send with
Response.Filter.Length
No need to metion, that you get the filename of the call using the
HttpContext.Current.Request.Path
This functions called with every single request, so you can get your size and you do the rest.
Here must note, that you need first to test this idea to see if its work, and maybe improve it, and have in mine that if you have compress the pages on server the length is not the correct and maybe you need to compress it on Global.asax to have the actually lenght.
Hope this help.