Make a dialog box no larger than necessary and initially place it in the center of the screen.
Keep in mind, not all users have the same screen resolution that you might be using on your development workstation. Design all your dialogs for a minimum resolution of 1024x768 and remember to verify that your dialog boxes display properly at lower resolutions.
By default, AutoCAD initially displays all dialog boxes in the center of the graphics window. However, you can specify that dialog boxes display at an alternate location (such as the last location specified by the user). The new_dialog and done_dialog AutoLISP functions provide for dialog box placement.