I know this is an old topic, but just for the record... The same thing seems to happen on macOS as well (Cinema 4D R21.207, macOS Catalina 10.15.3). If xpos and ypos are -2 (i.e. show the dialog in the middle of the screen), and pluginID is not given as parameter, the dialog dimensions are about half of what they're supposed to be. If a pluginID is given, dialog dimensions are correct.