[DefaultValue(0L)]
public virtual long MaxFileSize { get; set; }

<DefaultValue(0L)>
Public Overridable Property MaxFileSize As Long

<dx:ASPxUploadControl ID="ASPxUploadControl1" runat="server" OnFileUploadComplete="uc_FileUploadComplete">
    <ValidationSettings AllowedFileExtensions=".txt,.jpg,.jpe,.jpeg,.doc" MaxFileSize="1000000" 
        FileDoesNotExistErrorText="A file cannot be found." 
        GeneralErrorText="File uploading fails due to an external error that doesn't relate to the ASPxUploadControl's functionality." 
        MaxFileSizeErrorText="Uploaded file size exceeds the maximum file size." 
        NotAllowedFileExtensionErrorText="The file extension is not allowed.">
        <ErrorStyle BackColor="Yellow" ForeColor="Black" />
    </ValidationSettings>
</dx:ASPxUploadControl>

protected void uc_FileUploadComplete(object sender, DevExpress.Web.FileUploadCompleteEventArgs e) {
    if (e.IsValid) {
        ASPxUploadControl1.SaveAs(MapPath("Images/" + ASPxUploadControl1.FileName));
    }
}

Protected Sub uc_FileUploadComplete(ByVal sender As Object, ByVal e As DevExpress.Web.FileUploadCompleteEventArgs)
    If e.IsValid Then
        ASPxUploadControl1.SaveAs(MapPath("Images/" & ASPxUploadControl1.FileName))
    End If
End Sub