@Priyanka.
There is no way to check File size before upload in normal FileUpload and hence you need to upload the File first check its size and if it exceeds 10 MB show error.
And hence for uploading large files you will need to set maxRequestLength to Maximum.
And if you set maxRequestLength less than 10 MB it will show page cannot be displayed.
Also you can create a Custom page say Error.aspx and display custom error on it say "File size exceeded the limit.". For that you will need to set CustomErrors ON in Web.Config and specify an error page
<system.web>
<customErrors mode="On" defaultRedirect="Error.aspx">
</customErrors>
</system.web>