srossi,
You have to use your dialog's method OnSize.
Use class wizard to add method OnSize to the code. Within the code, you determine the resized size of the window by using GetWindowSize();
You also add member variables in class wizard for each of your button/editbox items like m_OK, m_cancel, m_edit1 etc.
Now you add code within OnSize to change the button sizes using SetWindowRgn() method.
Hope it works.
flame