When the minimum size of a window is too big to fit on the screen, MUI tries to reduce font sizes and does a new calculation. You should always design your windows to fit on a 640*200 screen with all fonts set to topaz/8.
When a window is closed (and you specified a MUIA_Window_ID), MUI remembers its position and size and uses these values during the next opening.
After setting MUIA_Window_Open to TRUE, you should test if MUI was able to open the window by getting the attribute again. If you don't and if this was the only window of your application, the user won't be able to do any input and your application will seem to hang.