Validate Image Size using CustomValidator Control

Try this code in server side of CustomValidator

protected void CustomValidator_ServerValidate(object source, ServerValidateEventArgs args)
{
if (IsValid)
{
int maxSize = 5 * 1024 * 1024;
if (FileUpload1.PostedFile.ContentLength > maxSize)
{
args.IsValid = false;
}
}
}

Good Luck