Hi Rajiv,
You can custom the file too large error message by property "FileTooLargeMsg", like below. So you can shows your own message.
<CuteWebUI:UploadAttachments ID="uploader1" runat="server" FileTooLargeMsg="custom error message">
<ValidateOption MaxSizeKB="1024" />
</CuteWebUI:UploadAttachments>
Regards,
Ken