Dialogs are measured in DLUs, dialog units. The size of a dialog unit is dependent on the size of the system font. The idea is that dialogs should keep their relative size no matter what system font is used. The easiest way to make sure a dialog fits at the minimum supported resolution is simple - switch your graphics card to this resolution, draw a dialog box, then use this dialog box as a maximum-size template for all the other boxes. You can find info on DLUs on MSDN, search for GetDialogBaseUnits.