From http://www.w3schools.com (Copyright Refsnes Data)

ASP Size Property

The Size property is used to return the size, in bytes, of the specified file or folder.

Syntax

FileObject.Size

FolderObject.Size

Example for the File object

<%
dim fs,f
set fs=Server.CreateObject("Scripting.FileSystemObject")
set f=fs.GetFile("c:\test.asp")
Response.Write("The size of test.asp is: ")
Response.Write(f.Size & " bytes.")
set f=nothing
set fs=nothing
%>

Output:

The size of test.asp is: 10556 bytes.

Example for the Folder object

<%
dim fs,fo
set fs=Server.CreateObject("Scripting.FileSystemObject")
set fo=fs.GetFolder("c:\test")
Response.Write("The size of the folder test is: ")
Response.Write(fo.Size & " bytes.")
set fo=nothing
set fs=nothing
%>

Output:

The size of the folder test is: 123456 bytes.

Complete File Object Reference