If your question is "How do I determine the size of the uploaded file?" then
you can use the following code:

Use the following in your .aspx file (don't forget to declare it as a
System.Web.UI.HtmlControls.HtmlInputFile in your codebehind file as well):

<INPUT id="fileUpload" type="file" size="100" runat="server">

Use the following properties in your codebehind to determine the file size
in bytes (my code is in VB.NET, but it should be similar in C#):

fileUpload.PostedFile.ContentLength

This is very simple code, just remember that the size is returned in bytes,
so it is possible that you may need to do some simple conversions. If you
have any problems let me know. Good Luck!