Hi nauna,

You can check in UploadComplete event.

C#

protected void AjaxFileUpload1_UploadComplete(object sender, AjaxControlToolkit.AjaxFileUploadEventArgs e)
{
    string savePath = MapPath("~/Files/" + e.FileName);      
    if (e.FileSize > 1024)
    {
        return;
    }
    AjaxFileUpload1.SaveAs(savePath);    
}

VB.Net

Protected Sub AjaxFileUpload1_UploadComplete(ByVal sender As Object, ByVal e As AjaxControlToolkit.AjaxFileUploadEventArgs)
    Dim savePath As String = MapPath("~/Files/" & e.FileName)
    If e.FileSize > 1024 Then
        Return
    End If
    AjaxFileUpload1.SaveAs(savePath)
End Sub

JavaScript

<script type="text/javascript">
    function UploadComplete(sender, args) {
        var filesize = args.get_fileSize();
        var fileId = args.get_fileId();
        var status = document.getElementById('AjaxFileUpload1_FileItemStatus_' + fileId);
        var container = document.getElementById('AjaxFileUpload1_FileInfoContainer_' + fileId);

        if (filesize > 1024) {
            document.getElementById('lblStatus').innerText = "error";
            if (status.innerText) {
                status.innerText = " (Error)";
            }
            if (status.textContent) {
                status.textContent = " (Error)";
            }
            container.style.color = 'Red';
        }
    }
</script>
<cc1:AjaxFileUpload ID="AjaxFileUpload1" AllowedFileTypes="jpg,jpeg" runat="server" MaximumNumberOfFiles="4"  
    OnUploadComplete="AjaxFileUpload1_UploadComplete" OnClientUploadComplete="UploadComplete" />