ASP.NET
A complete description for the ASP.NET version is available in the How to: Adjust the Size of Pop up Dialogs displayed in ASP.NET Applications topic.
WinForms
Check out the PopupSizeExample.Module.Win\Controllers\CustomizeFormSizeController.xx file and code comments in it. See also the following product documentation:
Concepts> Extend Functionality> Controllers
DevExpress.ExpressApp> Frame> TemplateChanged
DevExpress.ExpressApp> XafApplication> CustomizeTemplate
DevExpress.ExpressApp.Templates> ISupportStoreSettings> SettingsReloaded