Using ASPxPopupControl to dynamically load user control(ascx) to popup content, you found that size of popup control is auto-resized to fit the size of loaded control (see example from devexpress code central - E346).
This is because we get html code of ascx control using ASPxCallback.GetRenderResult, which is later set to popup control using SetContentHtml function - this will internally set innerHTML of div with html code of our user control (ascx).
Using ASPxPopupControl to dynamically load aspx page (not user control) to popup content using function SetContentUrl, you have problem - NO auto-resize of popup content area to respect size of loaded aspx page within popup iframe.
Last weeks i try to do my best to make auto-resizing feature when popup control loads aspx page using SetContentUrl.
Finally i have an solution with such feature as an advanced example demo.
You can download example demo from
Please feel free to add comment if you have any question.